fix(ci): make the CI timing report unflakeable (#59818)

The 'CI timing report' job is pure observability — it collects per-job/step
durations from the GitHub API after the run and publishes an HTML gantt
report + PR-vs-main timing diff. It gates nothing (all-checks-pass does not
include it), yet it could redden a PR: the script makes dozens of paginated
API calls with the shared repo GITHUB_TOKEN and had zero retry handling, so
a single 403 (rate-limit burst when several PRs run CI concurrently) failed
the job. Observed twice in a row on PR #59805.

- api_get(): retry 403/429/5xx and connection errors with exponential
  backoff, honoring Retry-After / X-RateLimit-Reset (max 5 attempts, 120s
  cap). Non-transient statuses (404 etc.) still fail fast.
- main(): exhausted retries raise TimingsUnavailable, caught to emit a
  degraded summary line + placeholder HTML artifact and exit 0 — a metrics
  collector must never fail the PR's checks. No timings JSON is written on
  the degraded path so an empty baseline can never be cached.
- ci.yml: baseline-save steps on main skip gracefully when no JSON exists.

Verified with a mocked urlopen harness: retry-then-success (3 attempts),
exhausted-retries -> TimingsUnavailable, 404 fails fast without retry,
degraded main() exits 0 with summary + placeholder and no JSON, and the
--from-json happy path is unchanged.

.github/workflows/ci.yml

@@ -221,10 +221,17 @@ jobs:
 
       - name: Save baseline cache (main only)
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'
-        run: cp ci-timings.json ci-timings-baseline.json
+        run: |
+          # Degraded runs (API rate-limited) produce no ci-timings.json —
+          # skip rather than fail, and never cache an empty baseline.
+          if [ -f ci-timings.json ]; then
+            cp ci-timings.json ci-timings-baseline.json
+          else
+            echo "No timings JSON this run — skipping baseline update"
+          fi
 
       - name: Upload baseline to cache (main only)
-        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main' && hashFiles('ci-timings-baseline.json') != ''
         uses: actions/cache/save@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
         with:
           path: ci-timings-baseline.json