fix: account polling elapsed time on the monotonic clock

[yzxcj797]

Fixes #203.

Summary

PollTimer measured elapsed duration with time.time() — a wall clock. NTP corrections, manual clock changes or VM suspend/restore move it:

• a forward adjustment can make an otherwise healthy deferred chat / collection indexing / video-generation poll time out immediately (the issue's repro: a 9900s jump yields TimeoutError: Polling timed out after 9900.0s on a poll that had run for ~0s);
• a backward adjustment silently extends polling beyond the caller's requested timeout.

Both the sync and async polling paths share this helper. The fix uses time.monotonic() — which cannot go backward — for the start timestamp and the elapsed-time computation. Internal change only: no public API and no normal-case timing difference.

Testing

New tests/poll_timer_test.py (deterministic, mocks only the clock):

• forward jump — the issue's exact shape (10s timeout, 20s interval, wall clock leaps between construction and the first sleep computation): returns 9.5 instead of raising;
• backward jump with real elapsed time — 15s actually elapse while the wall clock moves back: the poll still times out instead of quietly polling forever;
• timeout on genuine elapsed time — pins that real timeouts still raise with the correct message.

3/3 pass with the fix; differential with poll_timer.py reverted fails all three (the first returns the full interval, the others never raise).