fix(mcp): reconnect Streamable HTTP after MCP server restart

[schuellerf]

…without bogus SSE fallback

When the remote MCP process restarts, Langflow kept a stale ClientSession: tool list updates could fail with "Session terminated", the session health check could re-raise instead of discarding the session, and any Streamable HTTP failure triggered SSE fallback and could lock transport preference to SSE—masking the real issue and producing misleading TaskGroup/dual-transport errors.

This invalidates all sessions for the server URL on list_tools failure, treat any health-check failure as a dead session, retry transient Streamable HTTP errors before considering SSE, only fall back to SSE for clear transport-mismatch signals (e.g. 404/405/406), reset and re-record transport preference accordingly, and bust sessions on run_tool using the same termination/connection signals.

Add unit tests with mocked streamablehttp_client and sse_client.

Please check if this makes sense and if we should do those retries…

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 7d7ede57-52bb-42e4-bae5-39482b00e648

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[schuellerf]

This PR is too late for 1.9.0, I guess, but will be part of 1.10.0 (see #12777 )
ping me if I should backport this to 1.9.x …