fix(server): send JSON-RPC parse error when stdio transport fails to parse message

[Oxygen56]

Description

When the stdio transport cannot parse an incoming JSON-RPC message (e.g. a deeply-nested ping request that exceeds pydantic's recursion limit during validate_json), stdin_readersends a rawException` through the read stream.

Previously, BaseSession._receive_loop() only forwarded this Exception to the application layer via _handle_incoming() without sending a JSON-RPC error response through the write stream. This left the client`'s request permanently pending until a client-side timeout.

Root Cause

The flow for deeply-nested JSON on stdio:

1. stdin_reader calls jsonrpc_message_adapter.validate_json(line) → ValidationError("recursion limit exceeded")
2. The exception is caught and sent as a raw Exception through read_stream_writer.send(exc)
3. BaseSession._receive_loop() receives the Exception, calls _handle_incoming(exc), but never sends a JSON-RPC response
4. Client waits forever for a response that will never arrive

Fix

Catch the transport-level Exception in BaseSession._receive_loop() and send a proper JSON-RPC parse error (code: -32700) with id: null through the write stream, matching JSON-RPC spec §5 and the existing Streamable HTTP transport behaviour.

Before (broken):

Client sends deeply-nested JSON → server logs error internally → no response → client hangs

After (fixed):

Client sends deeply-nested JSON → server sends {"jsonrpc":"2.0","id":null,"error":{"code":-32700,"message":"Parse error"}} → client can handle error

Changes

• src/mcp/shared/session.py: Added PARSE_ERROR import; added JSON-RPC error response emission in the isinstance(message, Exception) branch of _receive_loop()

The _handle_incoming(message) call is preserved so the application layer can still observe transport-level failures.

Fixes #2751