refactor(mcp): remove redundant type field from Content implementations (#27)

- The type field and associated methods were redundant in Content implementations (TextContent, ImageContent, EmbeddedResource)
  as the type information is already handled by Jackson's polymorphic type handling via @JsonSubTypes annotation.
- Add default implementation that returns the appropriate type string based on the implementing class (text, image, or resource)
- Added comprehensive unit tests for McpSchema to validate serialization/deserialization behavior of all schema components.
- Add deserialization tests for all content types
- Add json-unit-assertj for flexible JSON testing -  ignores array order and extra array items, reducing test brittleness while
maintaining functional validation.

Resolves #26
Resolve https://github.com/spring-projects/spring-ai/issues/2350

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

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

@@ -964,7 +964,18 @@ public final class McpSchema {
 			@JsonSubTypes.Type(value = EmbeddedResource.class, name = "resource") })
 	public sealed interface Content permits TextContent, ImageContent, EmbeddedResource {
 
-		String type();
+		default String type() {
+			if (this instanceof TextContent) {
+				return "text";
+			}
+			else if (this instanceof ImageContent) {
+				return "image";
+			}
+			else if (this instanceof EmbeddedResource) {
+				return "resource";
+			}
+			throw new IllegalArgumentException("Unknown content type: " + this);
+		}
 
 	}
 
@@ -972,19 +983,10 @@ public final class McpSchema {
 	public record TextContent( // @formatter:off
 		@JsonProperty("audience") List<Role> audience,
 		@JsonProperty("priority") Double priority,
-		@JsonProperty("type") String type,
 		@JsonProperty("text") String text) implements Content { // @formatter:on
 
-		public TextContent {
-			type = "text";
-		}
-
-		public String type() {
-			return type;
-		}
-
 		public TextContent(String content) {
-			this(null, null, "text", content);
+			this(null, null, content);
 		}
 	}
 
@@ -992,33 +994,15 @@ public final class McpSchema {
 	public record ImageContent( // @formatter:off
 		@JsonProperty("audience") List<Role> audience,
 		@JsonProperty("priority") Double priority,
-		@JsonProperty("type") String type,
 		@JsonProperty("data") String data,
 		@JsonProperty("mimeType") String mimeType) implements Content { // @formatter:on
-
-		public ImageContent {
-			type = "image";
-		}
-
-		public String type() {
-			return type;
-		}
 	}
 
 	@JsonInclude(JsonInclude.Include.NON_ABSENT)
 	public record EmbeddedResource( // @formatter:off
 		@JsonProperty("audience") List<Role> audience,
 		@JsonProperty("priority") Double priority,
-		@JsonProperty("type") String type,
 		@JsonProperty("resource") ResourceContents resource) implements Content { // @formatter:on
-
-		public EmbeddedResource {
-			type = "resource";
-		}
-
-		public String type() {
-			return type;
-		}
 	}
 
 	// ---------------------------