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

[schuellerf]

This is the rebase of #12270 onto 1.10.0 as #12270 never seemed to have landed in 1.9.0

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This PR contains the fix to reconnect streamable HTTP after MCP server restart 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…

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[schuellerf]

@erichare am I right, that it makes sense to merge this to 1.10.0 as 1.9.0 is already released? (so just close #12270 )
Additionally to the question if this PR is reproducible on your end and makes sense, of couse :-D

[erichare]

Overall LGTM — the transient-vs-transport-mismatch split is the right model, invalidate_server_key cleanly fixes the stale-session bug, and tests cover the key paths. No blockers from my read — merge-worthy.

Two things worth addressing (non-blocking, could be follow-ups):

1. Substring matching on str(exc) for "404/405/406" in _should_attempt_sse_after_streamable_failure will false-positive on unrelated errors whose message happens to contain those digits, silently flipping to SSE. Prefer typed checks only, or word-boundary match.

2. _validate_session_connectivity now catches bare Exception. Solves the real bug, but a targeted widening (McpError, ClosedResourceError, BaseExceptionGroup) would avoid masking unrelated errors.

Nits: _iter_exception_leaves can use plain import builtins; invalidate_server_key is public but callers reach into private _get_server_key — consider a public helper.