Two unit tests that pin down the threading.local semantics the CLI freeze
fix (#13617 / #13618) relies on:
- main-thread registration must be invisible to child threads (documents
the underlying bug — if this ever starts passing visible, ACP's
GHSA-qg5c-hvr5-hjgr race has returned)
- child-thread registration must be visible from that same thread AND
cleared by the finally block (documents the fix pattern used by
cli.py's run_agent closure and acp_adapter/server.py)
Pairs with the fix in the preceding commit by @Societus.
