ci: pr-test-summary job shows red when the GitHub API returns persistent 5xx errors #2256

Problem

The PR test summary (non-blocking) job in pr.yaml sometimes fails with a red check on PRs, for example:

Traceback (most recent call last):
  File "/home/runner/work/cuopt/cuopt/ci/utils/pr_test_summary.py", line 325, in <module>
    main()
...
urllib.error.HTTPError: HTTP Error 502: Bad Gateway

This was noticed in the discussion on #2241 (run https://github.com/NVIDIA/cuopt/actions/runs/41135602235). It was first mistaken for leftover RAPIDS CI telemetry, but it comes from this repo's own ci/utils/pr_test_summary.py.

Why it fails

pr_test_summary.py makes several GitHub API calls: it lists the run's jobs, downloads logs of failed jobs, and creates or updates the PR comment.

• Transient errors (429, 5xx, network errors) are retried up to 4 attempts with 2**attempt second backoff (ci: retry transient GitHub API errors in pr-test-summary #2204). That is roughly 14 seconds in total. If the API keeps returning errors for longer, the last exception is re-raised.
• The script then exits non-zero, and the run: python3 ci/utils/pr_test_summary.py step has no continue-on-error. ci: make pr-test-summary and NVSkills CI checks non-blocking #1696 removed it and instead excluded the job from the pr-builder merge gate (and checks has ignored_pr_jobs: "pr-test-summary").
• The job is therefore not a merge gate, but it still shows as a failed check on the PR. That is confusing for contributors and reviewers, who may take it for a real test failure, and the summary comment is not posted.

Proposal

Make the summary best-effort so it cannot show red because of an API error:

• Catch persistent API errors at the end of main(), print a warning to the job log, and exit 0.
• Or set continue-on-error: true on the step.

Real script bugs (for example parsing errors) should still fail the job, so only the network and HTTP error classes would be swallowed.

Related: #2204, #1696, #2241.