Fixes #8636 (and likely the same root cause as #7255 and #7220). Three agent tests have been reported as flaky on loaded CI machines: `test_self_stat_collection_count` (#8636, #4657), `test_self_stat_error_counter` (#7255), and `verify_producer_details` (#7220). A fourth, `test_self_stat_unreachable_counter`, uses the same pattern and was likely susceptible too. They all shared the same approach: pause tokio time, advance by a fixed `TEST_WAIT_PERIOD`, then assert. On slow machines, the collection task couldn't keep up with the timer: its bounded channel (capacity 1) would overflow when the next tick fired before the previous collection finished, recording spurious `CollectionsInProgress` failures. The fix introduces `advance_n_collections()`, which advances paused time in small increments (`TICK_INTERVAL`), using a watch channel to wait for each collection to complete before moving on to the next. This guarantees the channel is drained before the next timer tick fires, so collections never overlap. Because collections can no longer overlap, `test_self_stat_error_counter` gets substantially simpler: the old code had to tolerate both `CollectionsInProgress` and `500` failure reasons (summing across them) and allowed `+1` slack between the server-side and task-side counts. Now it can assert exactly `N_COLLECTIONS` errors of a single type and exact count equality. `test_updated_producer_is_still_collected_from` also gets a more precise wait loop after re-registration: instead of advancing time by a fixed amount and hoping a collection happened, it watches for the server-side `collection_count` to confirm an actual collection from the new server (as opposed to just a producer info update from re-registration). Also adds `CollectionTaskHandle::details_watcher()` (`#[cfg(test)]`) to expose the watch receiver.

Test flake: oximeter-collector agent::tests::test_self_stat_collection_count #4657

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