[2Zest]

Fixes #3799.

When a Streamable HTTP request carries a session ID but the server responds with HTTP 404, raise a dedicated SessionNotFoundError while preserving the original JSON-RPC code, message, and data. Normal 404 responses without an active session continue to use the existing MCPError behavior.

Added regression coverage for session termination responses, including 204 DELETE termination flow.

Validation: python -m compileall -q src/mcp tests/shared/test_streamable_http.py and git diff --check. Runtime pytest could not be run in this environment because pytest and project dependencies are not installed.

[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 #3799.

If a maintainer assigns you to #3799, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

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

• We're a small team with very little capacity to review community PRs right now.
• Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.