fix(agents): offload UploadsMiddleware uploads scan off the event loop (#3311)

UploadsMiddleware defines only the sync `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 the
per-message uploads-directory scan (`exists`/`iterdir`/`stat` plus reading
sibling `.md` outlines) blocks the asyncio event loop on every message that has
an uploads directory.

Add `abefore_agent` that offloads the scan to a worker thread via
`run_in_executor`; it copies the current context, preserving the `user_id`
contextvar read by `get_effective_user_id()`.

Add a runtime anchor under `tests/blocking_io/` that drives the real
`create_agent` graph via `ainvoke` under the strict Blockbuster gate, so a
regression back onto the event loop fails CI. Update blocking-IO docs.

backend/docs/BLOCKING_IO_DETECTION.md

@@ -144,6 +144,9 @@ The runtime anchors protect confirmed blocking-IO bug shapes:
   (fix #3084); this anchor drives the real async API under the gate so any
   blocking IO reintroduced on the loop fails, not only removal of one
   `to_thread` call.
+- `UploadsMiddleware.before_agent` uploads-directory scan: a sync-only middleware
+  hook runs on the event loop under async graph execution, so the scan is
+  offloaded via `abefore_agent` + `run_in_executor`.
 - Gate health checks: Blockbuster catches unoffloaded calls, opt-out works, and
   patches are restored after exceptions.