Port session_store path to anyio so it works under trio

[dsmilkov]

The bug

The SDK's core (_internal/query.py, transport/subprocess_cli.py) is
anyio-based and works on both asyncio and trio — it even ships
_internal/_task_compat.spawn_detached precisely so background tasks work
on either backend. But the session_store code path (TranscriptMirrorBatcher,
parts of session_resume.py, and _derive_infos_via_load in sessions.py)
uses raw asyncio.ensure_future / asyncio.Lock / asyncio.wait_for /
asyncio.sleep / asyncio.Semaphore / asyncio.gather.

Under a trio app, passing session_store= to query() or ClaudeSDKClient
crashes at the first batcher flush with:

TypeError: trio.run received unrecognized yield message <Task pending ...>

(an asyncio.Task yielded to trio's event loop).

The fix

Port those three modules to anyio, reusing the SDK's existing spawn_detached:

• transcript_mirror_batcher.py — anyio.Lock, anyio.sleep,
  anyio.fail_after; eager-flush fires via spawn_detached(self._drain())
  (handle stored in _flush_task: TaskHandle | None); flush() simply
  awaits _drain() (the lock already serializes with in-flight eager
  drains, so the old Task + compare-and-null dance was redundant);
  close() shields its final flush so the last batch lands even when
  teardown runs under a cancelled scope. _swallow_done_exception is
  removed — spawn_detached handles exception surfacing on both backends.
• session_resume.py — _with_timeout → anyio.fail_after;
  _rmtree_with_retry → anyio.sleep + anyio.get_cancelled_exc_class().
  The synchronous happy-path rmtree is preserved so cleanup still runs
  under a cancelled scope.
• sessions.py — _derive_infos_via_load →
  anyio.CapacityLimiter + anyio.create_task_group() with per-task
  try/except Exception, preserving the "one adapter failure degrades
  that row" semantics.

Behavior

• asyncio callers: no intended change — anyio dispatches to native
  asyncio primitives; spawn_detached on asyncio is loop.create_task().
• trio callers: now works.
• Two deliberate narrowings (noted in case anyone relied on the old edge):
  • On Py3.10 only, an adapter's append() that raises
    asyncio.TimeoutError from its own internals is now retried
    (previously short-circuited). On 3.11+ asyncio.TimeoutError is TimeoutError so there's no change. Adapter-internal timeouts aren't
    our send_timeout, so retrying them is arguably more correct.
  • _derive_infos_via_load no longer swallows BaseException
    (cancellation / KeyboardInterrupt) from a store.load() into an
    empty-summary row — it propagates.

Tests

• New tests/test_session_store_anyio.py: 7 tests × ["asyncio", "trio"],
  deliberately limited to backend-divergent paths — the existing per-module
  suites already cover behavior in depth under pytest-asyncio. Covered: the
  spawn_detached eager-flush path (the original crash site), eager-flush /
  flush() interleaving order, send-timeout reporting via fail_after,
  close() flushing under a cancelled scope, _with_timeout timeout
  conversion, _rmtree_with_retry under a cancelled scope, and
  list_sessions_from_store with one failing load.
• tests/test_transcript_mirror.py updated for the removed
  _swallow_done_exception, the anyio.sleep patch target, and the
  TaskHandle type of _flush_task.
• Full suite: 805 pass, 3 skip (live-only). mypy --strict clean, ruff clean.

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❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
src/claude_agent_sdk/_internal/session_resume.py	83.33%	1 Missing ⚠️
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