fix(client): map HTTP 401 on streamable HTTP to Unauthorized JSON-RPC error

[mturac]

Summary

Maps bare HTTP 401 responses on streamable HTTP POST requests to a distinguishable JSON-RPC error (INVALID_REQUEST / "Unauthorized" with data.http_status) instead of collapsing into the generic "Server returned an error response" fallback.

This lets agents and other callers handle operation-specific auth denials without tearing down the whole session.

Changes

• src/mcp/client/streamable_http.py: added explicit 401 handling in _handle_post_request.
• tests/client/test_notification_response.py: added end-to-end session-level regression test.
• tests/client/test_streamable_http.py: added transport-level regression test.

Fixes #1295

AI assistance disclosure

AI was used for analysis and implementation support; I reviewed and verified the changes and tests.

[github-actions]

This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #1295.

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CONTRIBUTING.md has the full reasoning, but in short:

• We're a small team with very little capacity to review community PRs right now.
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Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.