refactor: fix the return format of CallToolResult

- corrected the return format for structured content results, following the MCP protocol.
- fix variable name

Author: TheEterna

mcp-annotations/src/main/java/org/springaicommunity/mcp/method/tool/AbstractMcpToolMethodCallback.java

@@ -23,8 +23,8 @@
 import java.util.Objects;
 import java.util.stream.Stream;
 
-import io.modelcontextprotocol.spec.McpSchema.CallToolRequest;
-import io.modelcontextprotocol.spec.McpSchema.CallToolResult;
+import io.modelcontextprotocol.spec.McpSchema.*;
+import io.modelcontextprotocol.spec.McpSchema.CallToolResult.*;
 import org.springaicommunity.mcp.annotation.McpMeta;
 import org.springaicommunity.mcp.annotation.McpProgressToken;
 import org.springaicommunity.mcp.annotation.McpTool;
@@ -144,36 +144,52 @@ protected Object buildTypedArgument(Object value, Type type) {
 	 * @return A CallToolResult representing the processed result
 	 */
 	protected CallToolResult convertValueToCallToolResult(Object result) {
+		Builder callToolResultBuilder = CallToolResult.builder();
+
+		// According to the MCP protocol For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.
+		if (this.returnMode == ReturnMode.STRUCTURED) {
+			String jsonOutput = JsonParser.toJson(result);
+			Object structuredOutput = JsonParser.fromJson(jsonOutput, Object.class);
+			callToolResultBuilder.structuredContent(structuredOutput);
+		}
+
 		// Return the result if it's already a CallToolResult
 		if (result instanceof CallToolResult) {
 			return (CallToolResult) result;
+		} else if (result instanceof TextContent textContent) {
+			// Structured content is only supported in TextContent
+			return callToolResultBuilder
+					.addContent(textContent)
+					.isError(false)
+					.meta(null)
+					.build();
+		} else if (result instanceof Content content) {
+			return CallToolResult.builder()
+					.addContent(content)
+					.isError(false)
+					.meta(null)
+					.build();
 		}
 
 		Type returnType = this.toolMethod.getGenericReturnType();
 
 		if (returnMode == ReturnMode.VOID || returnType == Void.TYPE || returnType == void.class) {
-			return CallToolResult.builder().addTextContent(JsonParser.toJson("Done")).build();
-		}
-
-		if (this.returnMode == ReturnMode.STRUCTURED) {
-			String jsonOutput = JsonParser.toJson(result);
-			Object structuredOutput = JsonParser.fromJson(jsonOutput, Object.class);
-			return CallToolResult.builder().structuredContent(structuredOutput).build();
+			return callToolResultBuilder.addTextContent(JsonParser.toJson("Done")).build();
 		}
 
 		// Default to text output
 		if (result == null) {
-			return CallToolResult.builder().addTextContent("null").build();
+			return callToolResultBuilder.addTextContent("null").build();
 		}
 
 		// For string results in TEXT mode, return the string directly without JSON
 		// serialization
 		if (result instanceof String) {
-			return CallToolResult.builder().addTextContent((String) result).build();
+			return callToolResultBuilder.addTextContent((String) result).build();
 		}
 
 		// For other types, serialize to JSON
-		return CallToolResult.builder().addTextContent(JsonParser.toJson(result)).build();
+		return callToolResultBuilder.addTextContent(JsonParser.toJson(result)).build();
 	}
 
 	/**

mcp-annotations/src/main/java/org/springaicommunity/mcp/provider/tool/SyncMcpToolProvider.java

@@ -120,9 +120,9 @@ public List<SyncToolSpecification> getToolSpecifications() {
 
 					var tool = toolBuilder.build();
 
-					boolean useStructuredOtput = tool.outputSchema() != null;
+					boolean useStructuredOutput = tool.outputSchema() != null;
 
-					ReturnMode returnMode = useStructuredOtput ? ReturnMode.STRUCTURED
+					ReturnMode returnMode = useStructuredOutput ? ReturnMode.STRUCTURED
 							: (methodReturnType == Void.TYPE || methodReturnType == void.class ? ReturnMode.VOID
 									: ReturnMode.TEXT);