fix(client): surface streamable HTTP 401 as JSON-RPC error

[Sehlani042]

Summary

• map bare Streamable HTTP 401 POST responses for JSON-RPC requests to a correlated JSON-RPC error instead of the generic fallback
• keep existing JSON-RPC error bodies, 404 pre-session, and session-terminated handling unchanged
• add a regression test for operation-specific authorization failures

Fixes #1295.

Verification

• RED: PATH=/tmp/codex-uv-0.9.5/bin:$PATH uv run pytest tests/client/test_streamable_http.py::test_bare_401_request_maps_to_unauthorized_jsonrpc_error -q failed with Server returned an error response vs Unauthorized
• GREEN: PATH=/tmp/codex-uv-0.9.5/bin:$PATH uv run pytest tests/client/test_streamable_http.py::test_bare_401_request_maps_to_unauthorized_jsonrpc_error tests/client/test_streamable_http.py -q
• PATH=/tmp/codex-uv-0.9.5/bin:$PATH uv run ruff check src/mcp/client/streamable_http.py tests/client/test_streamable_http.py
• PATH=/tmp/codex-uv-0.9.5/bin:$PATH uv run ruff format --check src/mcp/client/streamable_http.py tests/client/test_streamable_http.py
• PATH=/tmp/codex-uv-0.9.5/bin:$PATH uv run pyright src/mcp/client/streamable_http.py tests/client/test_streamable_http.py
• git diff --check

Note: the local Homebrew uv is 0.9.4 while this repo requires >=0.9.5, so verification used an isolated /tmp/codex-uv-0.9.5 binary.

[cubic-dev-ai]

No issues found across 2 files

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