fix: Setting the proper content type for errors.

[ehsavoie]

The expected content type is application/problem+json when there is an error and not application/json

Fixes #719 🦕

[gemini-code-assist]

Summary of Changes

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This pull request refines the API's error handling by ensuring that all error responses consistently use the application/problem+json content type, providing a standardized and machine-readable format for problem details. It also improves the serialization of successful JSON responses and expands test coverage to validate these content type changes across the application's various communication protocols.

Highlights

• Standardized Error Content Type: The Content-Type header for error responses has been updated from application/json to application/problem+json across both JSON-RPC and REST endpoints, aligning with RFC 7807 for problem details.
• Conditional Content Type for JSON-RPC: For JSON-RPC non-streaming responses, the Content-Type is now dynamically set to application/problem+json if an error is present, otherwise it remains application/json for successful responses.
• Improved JSON Serialization: The JSON serialization logic for success responses in the REST handler was updated to use alwaysPrintFieldsWithNoPresence() for JsonFormat.printer(), ensuring all fields, including those with default values, are explicitly present.
• Enhanced Test Coverage: Existing tests were updated to assert the correct Content-Type for both success and error responses, and new tests were added to specifically verify application/problem+json for JSON parsing and method not found errors in JSON-RPC.

Changelog

• reference/jsonrpc/src/main/java/io/a2a/server/apps/quarkus/A2AServerRoutes.java
  • Introduced a constant JSON_ERROR_CONTENT_TYPE for application/problem+json.
  • Updated the Content-Type header for direct error responses to application/problem+json.
  • Modified the Content-Type header for non-streaming responses to be application/problem+json if an error is present, otherwise application/json.
• reference/jsonrpc/src/test/java/io/a2a/server/apps/quarkus/A2AServerRoutesTest.java
  • Added static imports for CONTENT_TYPE and APPLICATION_JSON.
  • Introduced a constant JSON_ERROR_CONTENT_TYPE for application/problem+json.
  • Added verify statements to several existing tests to assert that the Content-Type header is application/json for successful responses.
  • Added two new test cases (testJsonParseError_ContentTypeIsProblemJson and testMethodNotFound_ContentTypeIsProblemJson) to specifically verify that error responses have a Content-Type of application/problem+json.
• transport/rest/src/main/java/io/a2a/transport/rest/handler/RestHandler.java
  • Introduced constants JSON_ERROR_CONTENT_TYPE and JSON_CONTENT_TYPE.
  • Updated HTTPRestResponse creation in deleteTaskPushNotificationConfiguration, createSuccessResponse, createErrorResponse, getExtendedAgentCard, and getAgentCard to use the new content type constants.
  • Changed JsonFormat.printer().includingDefaultValueFields() to JsonFormat.printer().alwaysPrintFieldsWithNoPresence() in createSuccessResponse.
• transport/rest/src/test/java/io/a2a/transport/rest/handler/RestHandlerTest.java
  • Modified numerous test assertions to verify that error responses now return application/problem+json as their Content-Type instead of application/json.

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[gemini-code-assist]

Code Review

This pull request correctly updates the content type for error responses to "application/problem+json" across both JSON-RPC and REST handlers, with updated tests reflecting these changes. However, a critical security issue was identified in the REST transport: error response bodies are constructed using manual string concatenation, which could lead to JSON injection vulnerabilities. It is strongly recommended to use a proper JSON library for serialization to ensure all output is correctly escaped. Additionally, I've provided suggestions to improve code quality by reducing constant duplication, leveraging standard library constants, and enhancing test robustness by adjusting constant visibility.