Clarify tool validation error messages (#1023)

- DefaultJsonSchemaValidator now emits a neutral message;
- Call sites in ToolInputValidator and the server output handlers
  prepend their own context prefix,
- Integration test assertions strengthened accordingly.

Closes #986

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

* address review comments

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

* address review comments

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

---------

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

mcp-json-jackson3/src/main/java/io/modelcontextprotocol/json/schema/jackson3/DefaultJsonSchemaValidator.java

@@ -75,8 +75,7 @@ public class DefaultJsonSchemaValidator implements JsonSchemaValidator {
 			// Check if validation passed
 			if (!validationResult.isEmpty()) {
 				return ValidationResponse
-					.asInvalid("Validation failed: structuredContent does not match tool outputSchema. "
-							+ "Validation errors: " + validationResult);
+					.asInvalid("Validation failed: JSON schema validation errors: " + validationResult);
 			}
 
 			return ValidationResponse.asValid(jsonStructuredOutput.toString());