* fix(gateway): resolve the agent factory off the event loop
start_run() called resolve_agent_factory() inline, which lazily imports
the full lead-agent assembly stack (middlewares, authz, MCP, jsonschema;
measured at ~8s cold on Windows, multi-second on slower filesystems).
start_run is the single choke point every run-creation path flows
through, so the first run after Gateway startup blocked the event loop
for the whole import, stalling every other request, stream and channel
on the same loop - the loop-stall family issue #5172 fixed for agent
construction itself (the checkpoint-state path already resolves through
the assembly pool).
Resolve the factory via asyncio.to_thread: the first call pays one
worker-thread hop for the import, later runs hit the cached module.
Regression test pins the contract with a probe module whose module-level
__getattr__ records the thread that performed the from-import.
* fix(gateway): isolate factory resolution in the assembly pool
Move import regressions into the strict blocking-I/O suite and cover default-executor isolation, request context, and failure-before-admission ordering.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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