test: Assert RequestList persists the expected request contents

[anxkhn]

Three of the RequestList persistence tests in
tests/unit/request_loaders/test_request_list.py build a persisted value in the
key-value store and then only assert persisted_data is not None. Each of their
docstrings/comments claims to verify the actual persisted request set, but the
contents were never checked, so a regression that persisted the wrong URLs, an
empty list where data was expected, or a malformed structure would still pass.

This adds one content assertion per test on top of the existing null check:

• test_persist_requests_key_with_sync_iterable: the persisted URLs equal the
  three input URLs (docstring: "persists request data").
• test_persist_requests_key_with_empty_iterator: persisted_data['requests']
  equals [] (comment: "empty requests were persisted").
• test_handle_invalid_url_with_persistence: the persisted URLs are exactly
  ['https://valid.placeholder.com'], so the invalid URL is excluded (comment:
  "the valid URL was persisted and the invalid one was not").

The existing assert persisted_data is not None lines are kept: they narrow the
type before the new subscripting.

No production code changes; this only strengthens existing assertions.

Issues

• No related open issue. Self-identified test gap.

Testing

• uv run pytest tests/unit/request_loaders/test_request_list.py -v -> 11 passed.
• uv run poe lint (ruff format check + ruff check) -> clean on the changed file.
• uv run poe type-check -> no new diagnostics from this change.
• Confirmed each new assertion is load-bearing: temporarily changing an expected
  value makes exactly that test fail, and reverting restores green.

Checklist

• CI passed