[Bug]: HTTP+JSON error responses use non-standard fields

[jmesnil]

What happened?

Summary

HTTP+JSON error responses use non-standard fields (error/message) instead of RFC 9457 Problem Details format — missing required type field with A2A error URI.

Requirement

• ID: HTTP_JSON-ERR-001
• Section: 11.6 — Error responses use RFC 9457 Problem Details format
• Level: MUST
• Spec: https://github.com/a2aproject/A2A/blob/0833a5f5fd1b715519c0aecf9e3055e3f9f38089/docs/specification.md#116-error-handling

Specification

HTTP error responses use RFC 9457 Problem Details format with Content-Type: application/problem+json.

For A2A-specific errors, the type field MUST use the URI from the mapping table in Section 5.4 (Error Code Mappings).

Expected behavior

A2A error responses must include a type field with the error URI from the spec's mapping table:

{
  "type": "https://a2a-protocol.org/errors/task-not-found",
  "title": "Task Not Found",
  "status": 404,
  "detail": "The specified task ID does not exist or is not accessible"
}

Actual behavior

The SUT returns Content-Type: application/problem+json correctly, but the body uses non-standard fields without a type field:

{
  "error": "io.a2a.spec.TaskNotFoundError",
  "message": "Task not found"
}

Reproducer

# GET a non-existent task — should return RFC 9457 Problem Details with 'type'
curl -s http://localhost:9999/tasks/tck-nonexistent-pd-test | python3 -m json.tool

# Actual:   {"error": "io.a2a.spec.TaskNotFoundError", "message": "Task not found"}
# Expected: {"type": "https://a2a-protocol.org/errors/task-not-found", "title": "Task Not Found", "status": 404, "detail": "..."}

TCK test

tests/compatibility/core_operations/test_error_handling.py::TestRestErrorStructure::test_error_uses_problem_details

Relevant log output

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