test: add ChangesFollower empty feed edge case test - #3

PR summary

Adds a unit test for an untested ChangesFollower edge case: a one-off (FINITE mode) run against a changes feed with zero pending changes. The first _changes response returns results: [] and pending: 0, and the test asserts the iterator terminates cleanly with zero changes instead of hanging or erroring. Existing FINITE tests all use at least one non-empty batch; this covers the pending == 0 fast-exit path in _ChangesFollowerIterator._request_callback on the very first request. Uses the existing runner helper with a timeout guard so a regression that hangs fails the test rather than blocking the suite.

Fixes: n/a (test-only addition)

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

No unit test covers a FINITE mode ChangesFollower run when the feed has no pending changes at all (empty database / already caught-up since sequence).

What is the new behavior?

test_start_one_off_empty_feed verifies that start_one_off() terminates immediately with zero changes when the first response has pending: 0 and empty results. All 40 tests in test_changes_follower.py pass and pylint remains 10.00/10.

Does this PR introduce a breaking change?

• Yes
• No

Other information

n/a

Link to Devin session: https://app.devin.ai/sessions/5abf9660e1f0483ab2e7f7094c58807f
Requested by: @eml2026