Map JSON-RPC errors to appropriate HTTP status codes

[AlisterBaroi]

When the A2AFastAPI server receives an invalid JSON-RPC request (e.g. missing required fields), the _handle_requests correctly validates the request, generates the proper JSON-RPC error response (e.g. -32602 Invalid parameters), but always returns it with HTTP 200 OK.

ERROR  a2a.server.apps.jsonrpc.jsonrpc_app  Failed to validate base JSON-RPC request
pydantic_core._pydantic_core.ValidationError: 1 validation error for SendMessageRequest
params.message.messageId
  Field required [type=missing, ...]

INFO:     $URL:$PORT - "POST / HTTP/1.1" 200 OK

WARNING  a2a.server.apps.jsonrpc.jsonrpc_app  Request Error (ID: 1): Code=-32602, Message='Invalid parameters'

The JSON-RPC error body is correct (-32602), but the HTTP transport layer reports 200 OK

What is expected to happen?

JSON-RPC validation errors should map to semantically appropriate HTTP status codes. A reasonable mapping would be:

JSON-RPC Error Code	Meaning	Suggested HTTP Status
-32700	Parse error (malformed JSON)	400 Bad Request
-32600	Invalid request (bad JSON-RPC structure)	400 Bad Request
-32602	Invalid params (missing/bad fields)	400 Bad Request
-32601	Method not found	404 Not Found or 400 Bad Request
-32603	Internal error	500 Internal Server Error

Successful JSON-RPC responses (including application-level errors like "task not found") should remain 200.