feat: Add builder pattern for McpError and mutate method for capabilities

- Add builder pattern to McpError for structured error creation with validation
- Deprecate McpError(Object) constructor in favor of builder approach
- Add mutate() method to server capabilities for creating modified copies

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

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

@@ -5,6 +5,7 @@
 package io.modelcontextprotocol.spec;
 
 import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse.JSONRPCError;
+import io.modelcontextprotocol.util.Assert;
 
 public class McpError extends RuntimeException {
 
@@ -15,6 +16,7 @@ public class McpError extends RuntimeException {
 		this.jsonRpcError = jsonRpcError;
 	}
 
+	@Deprecated
 	public McpError(Object error) {
 		super(error.toString());
 	}
@@ -23,4 +25,37 @@ public class McpError extends RuntimeException {
 		return jsonRpcError;
 	}
 
+	public static Builder builder(int errorCode) {
+		return new Builder(errorCode);
+	}
+
+	public static class Builder {
+
+		private final int code;
+
+		private String message;
+
+		private Object data;
+
+		private Builder(int code) {
+			this.code = code;
+		}
+
+		public Builder message(String message) {
+			this.message = message;
+			return this;
+		}
+
+		public Builder data(Object data) {
+			this.data = data;
+			return this;
+		}
+
+		public McpError build() {
+			Assert.hasText(message, "message must not be empty");
+			return new McpError(new JSONRPCError(code, message, data));
+		}
+
+	}
+
 }

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

@@ -523,6 +523,21 @@ public final class McpSchema {
 		public record ToolCapabilities(@JsonProperty("listChanged") Boolean listChanged) {
 		}
 
+		/**
+		 * Create a mutated copy of this object with the specified changes.
+		 * @return A new Builder instance with the same values as this object.
+		 */
+		public Builder mutate() {
+			var builder = new Builder();
+			builder.completions = this.completions;
+			builder.experimental = this.experimental;
+			builder.logging = this.logging;
+			builder.prompts = this.prompts;
+			builder.resources = this.resources;
+			builder.tools = this.tools;
+			return builder;
+		}
+
 		public static Builder builder() {
 			return new Builder();
 		}