fix: keep the newer SessionWaiter registration when an older one finishes

[L4XB]

Motivation / 动机

Fixes #9996. register_wait() can replace the USER_SESSIONS entry of a session_id with a newer SessionWaiter, but the older waiter's _cleanup() always ran USER_SESSIONS.pop(self.session_id, None). So when waiter A finished, timed out or was cancelled after waiter B had registered for the same session, B's entry was removed while B's future was still pending: trigger() could not find B any more and B could only time out. This happens whenever the same user starts an interactive flow twice.

Modifications / 改动点

• SessionWaiter._cleanup() only removes the registry entry if it still points at the waiter being cleaned up (USER_SESSIONS.get(self.session_id) is self). Removing the filter and stopping the controller are unchanged.
• New tests/test_session_waiter.py: registering A then B for the same session, stopping A keeps B registered and reachable (trigger() runs B's handler with B's controller), and B still cleans up after itself; plus the plain single-waiter cleanup case. The first test fails on master and passes with the fix.

Verification / 验证

uv run pytest tests/test_session_waiter.py   (2 passed)
ruff check / ruff format --check on the changed files

Checklist / 检查清单

• 我已经确认了我的更改不会引入新的错误 / I have confirmed my changes do not introduce new errors
• 我已经添加了必要的测试 / I have added the necessary tests
• 我已经更新了相关文档 (if needed) / I have updated the relevant documentation (if needed)

Summary by Sourcery

Prevent stale session waiters from removing newer registrations and leaving interactive flows unreachable.

Bug Fixes:

• Preserve a newer session waiter registration when an older waiter finishes, times out, or is cancelled for the same session.
• Ensure single-waiter registrations are still removed during cleanup.

Tests:

• Add coverage for replacement waiter cleanup, triggering the newer waiter, and normal single-waiter cleanup.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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[kilisamemarisaaa]

Reviewed the latest head and the registry race fix is correct.
egister_wait() can replace USER_SESSIONS[session_id], while the older waiter's cleanup previously removed the newer entry unconditionally; the identity guard now preserves the active waiter and still cleans up the single-waiter case. The added tests exercise replacement cleanup, trigger reachability/handler controller identity, and normal cleanup, with fixture isolation for USER_SESSIONS and FILTERS. I found no remaining correctness blockers.