test: Deflake webhook dispatches iterate test by polling for the dispatch

[vdusek]

test_webhook_dispatches_iterate drained dispatches().iterate() once, right after webhook_client.test(). The dispatch is recorded asynchronously, so under load the listing can still be empty (total=0) and the test fails with assert 0 >= 1 (failed run). The sibling test_webhook_dispatches failed the same way twice before and got a poll in #1004; the iterate variant did not.

The test now keeps the dispatch returned by test() and drains the iterator via collect_iterate_until_present until that ID shows up, then asserts membership, which is stricter than the previous "at least one dispatch" check.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.27%. Comparing base (779b17f) to head (90c4b9c).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1047   +/-   ##
=======================================
  Coverage   95.27%   95.27%           
=======================================
  Files          59       59           
  Lines        5502     5502           
=======================================
  Hits         5242     5242           
  Misses        260      260           

Flag	Coverage Δ
integration	92.22% <ø> (-0.11%)	⬇️
unit	87.36% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.