fix: use application/json Content-Type for HTTP+JSON error responses

[jmesnil]

The spec is not explicit on that but the error responses should all use application/json.

• in JSON-RPC, an error response is a regular JSON-RPC response
• in HTTP+JSON, the error payload was moved from RFC 9457 to AIP-193 so the RFC-9457's application/problem+json no longer makes sense.

This fixes #768

[gemini-code-assist]

Summary of Changes

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This pull request standardizes the Content-Type header for error responses in both JSON-RPC and HTTP+JSON interactions to application/json. This change aligns the system with current best practices and updated specifications, moving away from application/problem+json which is no longer relevant for these error payloads. The update ensures consistent and correct content type reporting for all error scenarios.

Highlights

• Content-Type Standardization: Standardized the Content-Type header for HTTP+JSON error responses to application/json across the application.
• Removal of application/problem+json: Removed the APPLICATION_PROBLEM_JSON constant and all its references, aligning with updated specifications where application/problem+json is no longer relevant for these error payloads.
• Test Updates: Updated various test cases to reflect the new application/json content type for error responses, including renaming test methods and modifying assertions.

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[gemini-code-assist]

Code Review

This pull request standardizes the Content-Type for error responses across JSON-RPC and REST transports to application/json, removing the APPLICATION_PROBLEM_JSON media type. This involves updating MediaType.java, removing related imports, modifying Content-Type headers in error handling logic, and adjusting corresponding test assertions. The review feedback highlights an opportunity to improve maintainability by replacing hardcoded "application/json" strings with the APPLICATION_JSON constant in test files, specifically in A2AServerRoutesTest.java and RestHandlerTest.java.

[jmesnil]

/gemini review

[gemini-code-assist]

Code Review

This pull request standardizes the content type for all JSON responses to application/json, removing the distinction previously made with application/problem+json. This change involves removing the APPLICATION_PROBLEM_JSON constant from io.a2a.common.MediaType, updating A2AServerRoutes and RestHandler to consistently use APPLICATION_JSON for both success and error responses, and refactoring associated test files to reflect these new content type expectations. Minor code style and Javadoc updates were also included.