Add retries and timeout to Grafana request in flaky test detection

[schizophrenicmaniac]

The flaky test detection job calls the Grafana alertmanager API with a plain requests.get. That call has no timeout and no retry, so any blip on metrics.beam.apache.org fails the whole run. This change uses a session with urllib3 Retry: 5 retries with backoff on connection errors and 429/5xx responses, plus a 30s timeout. If Grafana is still failing after that, the run fails as it did before.

get_workflow_issues() also called issue.get_labels() for every open and closed flaky_test issue, which is one extra API request per issue. The labels already come with the issue listing, so it now reads issue.labels.

I added test_flaky_test_detection.py next to the existing tool tests. It covers alert parsing, retrying after a 503, no retry on a 404, the timeout, and the label lookup. The Grafana tests run against a small local HTTP server, so the real retry path is exercised.

Fixes #40662

[github-actions]

📋 Review Summary

This PR fixes a TypeError in flaky_test_detection.py caused by GitHub returning None for the body of empty issues, and optimizes label checks by inspecting the already-loaded issue.labels property instead of issuing extra API requests via issue.get_labels(). The fix is clean, directly addresses the failure, and includes comprehensive unit test coverage for the affected issue-parsing logic.

🔍 General Feedback

• The null-safe fallback (issue.body or "") and the switch from issue.get_labels() to issue.labels are idiomatic and well-tested in test_flaky_test_detection.py.
• No blocking issues or regressions were found.

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[github-actions]

Assigning reviewers:

R: @Abacn added as fallback since no labels match configuration
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