Fix Streamable HTTP Accept negotiation

[raashish1601]

Summary

• accept either application/json or text/event-stream for Streamable HTTP POSTs when SSE-capable mode is enabled
• negotiate JSON only when the client does not accept SSE, while preserving SSE when both media types are allowed
• cover JSON-only and SSE-only clients in the shared transport tests and keep the race-condition regression focused on truly invalid Accept headers

Fixes #2349

Testing

• $env:PYTHONPATH='src'; python -m pytest tests/shared/test_streamable_http.py -k "accept_header_validation or accept_header_wildcard or single_media_type_negotiates_response or accept_header_incompatible or json_response_accept_json_only"
• $env:PYTHONPATH='src'; python -m pytest tests/issues/test_1363_race_condition_streamable_http.py -k invalid_accept_headers
• $env:PYTHONPATH='src'; python -m pytest tests/shared/test_streamable_http.py -k "test_response or test_json_response or server_backwards_compatibility_no_protocol_version"
• python -m ruff check src/mcp/server/streamable_http.py tests/shared/test_streamable_http.py tests/issues/test_1363_race_condition_streamable_http.py