fix(code): stop leaking turn coroutines and sqlite handles

[Mason Daugherty (mdrxy)]

Interrupting a turn, or exiting while a background thread-cache refresh is in flight, no longer strands an unawaited coroutine or an open SQLite handle.

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The unit suite emitted three families of warnings, all at teardown. Two are ours and are fixed here; the third is upstream and is filtered.

An unawaited turn coroutine

_send_to_agent handed run_worker an already-built _run_agent_task coroutine. Textual never runs the work of a worker cancelled before its first event-loop step, so that coroutine was finalized unawaited — a RuntimeWarning, and (once interpreter teardown had gone far enough to break the import machinery its cleanup relies on) an unraisable KeyError: '__import__'. Passing a callable means the coroutine only exists if the worker actually runs, so there is nothing to strand. Several tests were closing that coroutine by hand purely to silence the warning; those workarounds are gone, and a new test pins the callable contract.

Where it started: #5196. Handing run_worker a coroutine dates all the way back to the original Textual REPL (#686), but it was harmless until something cancelled a worker before its first step. #5196 added the recovery path for exactly that situation along with the tests that exercise it. Its parent commit runs the app test module with zero never awaited warnings; #5196 itself produces three.

An unclosed SQLite handle

aiosqlite opens the database on its worker thread and hands the raw sqlite3.Connection back through a future, recording it on the connection only when the awaiting coroutine resumes. A cancel landing anywhere in that window left the handle unreachable from the cleanup that follows, so the garbage collector reported ResourceWarning: unclosed database. There are two halves to the window, and both are now covered:

• Cancelled while the worker is still opening, the library has no handle recorded yet, so the cleanup it queues closes nothing. The session module now records the handle from the worker thread the moment the connector returns.
• Cancelled after the handle is delivered but before the coroutine resumes, the library clears its own reference before that queued cleanup can run — so it again closes nothing. The guard now also queues an explicit close ahead of the library's cleanup, while the handle is still reachable.

Both closes run on the thread that opened the handle, and closing twice is a no-op, so neither disturbs a normal shutdown. get_checkpointer builds its connection through the same helper rather than AsyncSqliteSaver.from_conn_string, so it gets the same guard.

Where it started: #5174. The prewarm that reads the session database has existed since #1481, but it ran once at startup, so it had normally finished before anything cancelled it. #5174 re-fires it after every turn, which reliably leaves a session-DB read in flight when a test app exits. Counting handles that aiosqlite opened and never closed across the goal-command tests: zero on the parent commit, fourteen on #5174.

A typing deprecation from google-genai

google.genai.types builds a union alias out of typing._UnionGenericAlias, which CPython 3.14 deprecates, and it fires at import before any of this package's code runs. This one is not a regression from any change here — deepagents-code has been tested on the 3.14 leg since the package was created in #3027, and the warning appears wherever a test imports the Google integration. It is tracked upstream as googleapis/python-genai#1640 and still unfixed as of google-genai 2.13.0, so it is filtered narrowly (message, category, and module) rather than worked around.

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