fix(agents): offload UploadsMiddleware uploads scan off the event loop

[ShenAC-SAC]

Problem

UploadsMiddleware defines only the synchronous before_agent hook. LangChain wires a sync-only hook as RunnableCallable(before_agent, None), and LangGraph's ainvoke runs it directly on the event loop when afunc is None. So before_agent's uploads-directory scan (exists / iterdir / stat + reading sibling .md outlines) blocks the asyncio event loop on every message that has an uploads directory.

Fix

Add abefore_agent that offloads the scan to a worker thread via run_in_executor. run_in_executor copies the current context, so the user_id contextvar read by get_effective_user_id() is preserved. The sync before_agent path (used when the graph runs synchronously) is unchanged.

Test

Add a runtime anchor under backend/tests/blocking_io/ that drives the real create_agent graph via ainvoke under the strict Blockbuster gate:

• Without the fix it fails with BlockingError: Blocking call to os.stat inside before_agent (verified).
• With the fix it passes.
• make detect-blocking-io no longer reports the SYNC_AGENT_MIDDLEWARE_HOOK finding for this path.

Docs (backend/CLAUDE.md, backend/docs/BLOCKING_IO_DETECTION.md) updated to list the new anchor.

Closes #3310

[Copilot]

Pull request overview

Adds an async abefore_agent hook to UploadsMiddleware that offloads its blocking uploads-directory scan to a worker thread via run_in_executor, preventing event-loop stalls under async graph execution. A blocking-IO runtime anchor and docs updates are included.

Changes:

• Implement abefore_agent in UploadsMiddleware delegating to before_agent via run_in_executor (preserves context).
• Add tests/blocking_io/test_uploads_middleware.py driving the real create_agent graph via ainvoke under the Blockbuster gate.
• Update BLOCKING_IO_DETECTION.md and CLAUDE.md to document the new anchor.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
backend/packages/harness/deerflow/agents/middlewares/uploads_middleware.py	Adds async hook offloading sync before_agent via run_in_executor.
backend/tests/blocking_io/test_uploads_middleware.py	New runtime anchor under Blockbuster gate using real create_agent graph.
backend/docs/BLOCKING_IO_DETECTION.md	Documents UploadsMiddleware runtime coverage.
backend/CLAUDE.md	Lists the new blocking-IO anchor alongside existing ones.

[WillemJiang]

@ShenAC-SAC Please fix the conflict with the main branch.

[ShenAC-SAC]

@WillemJiang Thanks for the heads-up — conflict resolved. I merged the latest main into this branch. The only conflicts were in backend/CLAUDE.md and backend/docs/BLOCKING_IO_DETECTION.md, where main's JsonlRunEventStore async-IO anchor (#3084) and this PR's UploadsMiddleware uploads-scan anchor landed in the same list; I kept both entries. The PR now merges cleanly. PTAL.