fix: support valid JSON value in tool structured content output (#551)

- Change CallToolResult.structuredContent type from Map<String,Object> to Object
  to support both objects and arrays
- Update JsonSchemaValidator to validate any Object type, not just Maps
- Add test cases for array-type structured output validation
- Fix type casting in existing tests to handle the more generic Object type
- Deprecate CallToolResult constructors in favor of builder pattern
- Update server implementations to use CallToolResult builder

This allows tools to return arrays as structured content, not just objects,
which is required by the MCP specification for tools with array-type output schemas.

It is realated to MPC spec fix: https://github.com/modelcontextprotocol/modelcontextprotocol/issues/834

Fixes #550

BREAKING CHANGE!

Migration Guide:
----------------
1. If you're accessing CallToolResult.structuredContent():
   - Add explicit casting when you know it's a Map:
     Before: result.structuredContent().get("key")
     After:  ((Map<String,Object>) result.structuredContent()).get("key")

   - Or check the type first:
     if (result.structuredContent() instanceof Map) {
         Map<String,Object> map = (Map<String,Object>) result.structuredContent();
         // use map
     } else if (result.structuredContent() instanceof List) {
         List<?> list = (List<?>) result.structuredContent();
         // use list
     }

2. If you're creating CallToolResult instances:
   - Switch to using the builder pattern:
     Before: new CallToolResult(content, isError, structuredContent)
     After:  CallToolResult.builder()
               .content(content)
               .isError(isError)
               .structuredContent(structuredContent)
               .build()

3. If you're implementing JsonSchemaValidator:
   - Update your validate() method signature:
     Before: validate(Map<String,Object> schema, Map<String,Object> structuredContent)
     After:  validate(Map<String,Object> schema, Object structuredContent)

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

mcp/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java

@@ -15,15 +15,9 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.function.BiFunction;
 
-import io.modelcontextprotocol.spec.DefaultMcpStreamableServerSessionFactory;
-import io.modelcontextprotocol.spec.McpServerTransportProviderBase;
-import io.modelcontextprotocol.spec.McpStreamableServerTransportProvider;
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
-
+import io.modelcontextprotocol.spec.DefaultMcpStreamableServerSessionFactory;
 import io.modelcontextprotocol.spec.JsonSchemaValidator;
 import io.modelcontextprotocol.spec.McpClientSession;
 import io.modelcontextprotocol.spec.McpError;
@@ -34,14 +28,17 @@ import io.modelcontextprotocol.spec.McpSchema.LoggingLevel;
 import io.modelcontextprotocol.spec.McpSchema.LoggingMessageNotification;
 import io.modelcontextprotocol.spec.McpSchema.ResourceTemplate;
 import io.modelcontextprotocol.spec.McpSchema.SetLevelRequest;
-import io.modelcontextprotocol.spec.McpSchema.TextContent;
 import io.modelcontextprotocol.spec.McpSchema.Tool;
 import io.modelcontextprotocol.spec.McpServerSession;
 import io.modelcontextprotocol.spec.McpServerTransportProvider;
+import io.modelcontextprotocol.spec.McpServerTransportProviderBase;
+import io.modelcontextprotocol.spec.McpStreamableServerTransportProvider;
 import io.modelcontextprotocol.util.Assert;
 import io.modelcontextprotocol.util.DeafaultMcpUriTemplateManagerFactory;
 import io.modelcontextprotocol.util.McpUriTemplateManagerFactory;
 import io.modelcontextprotocol.util.Utils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
@@ -420,8 +417,11 @@ public class McpAsyncServer {
 					// TextContent block.)
 					// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content
 
-					return new CallToolResult(List.of(new McpSchema.TextContent(validation.jsonStructuredOutput())),
-							result.isError(), result.structuredContent());
+					return CallToolResult.builder()
+						.content(List.of(new McpSchema.TextContent(validation.jsonStructuredOutput())))
+						.isError(result.isError())
+						.structuredContent(result.structuredContent())
+						.build();
 				}
 
 				return result;

mcp/src/main/java/io/modelcontextprotocol/server/McpStatelessAsyncServer.java

@@ -293,8 +293,11 @@ public class McpStatelessAsyncServer {
 					// TextContent block.)
 					// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content
 
-					return new CallToolResult(List.of(new McpSchema.TextContent(validation.jsonStructuredOutput())),
-							result.isError(), result.structuredContent());
+					return CallToolResult.builder()
+						.content(List.of(new McpSchema.TextContent(validation.jsonStructuredOutput())))
+						.isError(result.isError())
+						.structuredContent(result.structuredContent())
+						.build();
 				}
 
 				return result;

mcp/src/main/java/io/modelcontextprotocol/spec/DefaultJsonSchemaValidator.java

@@ -51,7 +51,7 @@ public class DefaultJsonSchemaValidator implements JsonSchemaValidator {
 	}
 
 	@Override
-	public ValidationResponse validate(Map<String, Object> schema, Map<String, Object> structuredContent) {
+	public ValidationResponse validate(Map<String, Object> schema, Object structuredContent) {
 
 		Assert.notNull(schema, "Schema must not be null");
 		Assert.notNull(structuredContent, "Structured content must not be null");

mcp/src/main/java/io/modelcontextprotocol/spec/JsonSchemaValidator.java

@@ -40,6 +40,6 @@ public interface JsonSchemaValidator {
 	 * @return A ValidationResponse indicating whether the validation was successful or
 	 * not.
 	 */
-	ValidationResponse validate(Map<String, Object> schema, Map<String, Object> structuredContent);
+	ValidationResponse validate(Map<String, Object> schema, Object structuredContent);
 
 }

mcp/src/main/java/io/modelcontextprotocol/spec/McpSchema.java

@@ -11,9 +11,6 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
-import org.slf4j.Logger;
-import org.slf4j.LoggerFactory;
-
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonInclude;
@@ -23,8 +20,9 @@ import com.fasterxml.jackson.annotation.JsonTypeInfo;
 import com.fasterxml.jackson.annotation.JsonTypeInfo.As;
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
-
 import io.modelcontextprotocol.util.Assert;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Based on the <a href="http://www.jsonrpc.org/specification">JSON-RPC 2.0
@@ -1508,15 +1506,21 @@ public final class McpSchema {
 	public record CallToolResult( // @formatter:off
 		@JsonProperty("content") List<Content> content,
 		@JsonProperty("isError") Boolean isError,
-		@JsonProperty("structuredContent") Map<String, Object> structuredContent,
+		@JsonProperty("structuredContent") Object structuredContent,
 		@JsonProperty("_meta") Map<String, Object> meta) implements Result { // @formatter:on
 
-		// backwards compatibility constructor
+		/**
+		 * @deprecated use the builder instead.
+		 */
+		@Deprecated
 		public CallToolResult(List<Content> content, Boolean isError) {
-			this(content, isError, null, null);
+			this(content, isError, (Object) null, null);
 		}
 
-		// backwards compatibility constructor
+		/**
+		 * @deprecated use the builder instead.
+		 */
+		@Deprecated
 		public CallToolResult(List<Content> content, Boolean isError, Map<String, Object> structuredContent) {
 			this(content, isError, structuredContent, null);
 		}
@@ -1551,7 +1555,7 @@ public final class McpSchema {
 
 			private Boolean isError = false;
 
-			private Map<String, Object> structuredContent;
+			private Object structuredContent;
 
 			private Map<String, Object> meta;
 
@@ -1566,7 +1570,7 @@ public final class McpSchema {
 				return this;
 			}
 
-			public Builder structuredContent(Map<String, Object> structuredContent) {
+			public Builder structuredContent(Object structuredContent) {
 				Assert.notNull(structuredContent, "structuredContent must not be null");
 				this.structuredContent = structuredContent;
 				return this;
@@ -1644,7 +1648,7 @@ public final class McpSchema {
 			 * @return a new CallToolResult instance
 			 */
 			public CallToolResult build() {
-				return new CallToolResult(content, isError, structuredContent, meta);
+				return new CallToolResult(content, isError, (Object) structuredContent, meta);
 			}
 
 		}