…#1926)

Unifies eventually-consistent / settling waits across the test suite
behind a single `poll_until_condition` helper, matching the form shared
with apify/apify-client-python#844.

## Changes

- **Single helper.** Replace `wait_for_condition` with
`poll_until_condition` (`tests/_utils.py`): it polls a sync-or-async
callable until an optional `condition` holds (default: truthy) or a
wall-clock `timeout` expires, then returns the last result so the caller
asserts explicitly. Exponential backoff is folded into a
`backoff_factor` argument instead of a separate `call_with_exp_backoff`.
- **Shared location + relative imports.** Move the helper from
`tests/unit/utils.py` to the shared `tests/_utils.py` and import it
relatively. This requires the test tree to be a proper package, so add
`__init__.py` to the `tests/unit` and `tests/e2e` subpackages that
lacked one (and allow `TID252` in tests).
- **Migrate hand-rolled waits.** Replace fixed `sleep`s and
`for`/`while` polling loops with the helper:
  - 5 autoscaling waits — `test_autoscaled_pool.py`
  - statistics-initialization loop — `test_basic_crawler.py`
  - 2 sitemap-loader waits — `test_sitemap_request_loader.py`
  - 2 request-queue background-task waits — `test_request_queue.py`

Lint, type-check, and the full unit suite (1883 passed) pass.

…flake via polling (#920)

## Summary

The SDK counterpart of apify/apify-client-python#844: introduce a single
shared `tests/_utils.py` with one polling helper, and migrate the whole
test suite to it — replacing fixed sleeps and hand-rolled retry loops
that caused flakiness.

## Changes

- **Shared `tests/_utils.py`.** One `poll_until_condition(fn,
condition=bool, *, timeout, poll_interval, backoff_factor)` helper —
identical to the one in apify-client-python#844 — polls a sync-or-async
callable until a condition holds or a wall-clock timeout expires.
`backoff_factor=2` subsumes the former `call_with_exp_backoff`;
`timeout=0` is the "call once" case. The `maybe_await` adapter,
`generate_unique_resource_name`, and the shared RSA crypto test keys
move here too. The per-package `tests/integration/_utils.py` and
`tests/e2e/_utils.py` are removed, and cross-test imports (e.g. the
crypto keys, previously imported from the `test_crypto` module) now go
through this single module.
- **Request queue tests.** ~50 polling call sites in
`test_request_queue.py` migrated to `poll_until_condition`. The
single/shared timeout is centralized in an `rq_poll_timeout` fixture
(`timeout=0` single, `timeout=30` shared), backed by an `rq_access_mode`
fixture — replacing the access-mode derivation block previously
copy-pasted into every test. The shared-mode request-ordering
relaxations from #931 are preserved on top.
- **Deflaked tests.** Fixed the flaky
`test_actor_adds_webhook_and_receives_event` e2e test (the client now
stays alive 5 s after `add_webhook`, and the unbounded `INITIALIZED`
startup loop becomes bounded polling) — replaces #930. Replaced the
`retry_counter` loops in `test_actor_charge.py` (4×) and fixed sleeps in
`test_actor_lifecycle`, `test_actor_key_value_store`, and
`test_apify_event_manager` with condition polling.

Fixed sleeps that are semantically required are intentionally kept:
negative checks ("event must NOT fire"), mtime-granularity checks,
simulated latency, and sleeps inside deployed Actor `main` bodies (where
the helper is unavailable).

Verified: lint, type-check, and the full unit suite pass;
integration/e2e require a platform token and run in CI.