ci: don't fail the build when the coverage artifact upload hits the storage quota

Every test step passed; only 'Upload coverage' failed with

    Failed to CreateArtifact: Artifact storage quota has been hit.
    Unable to upload any new artifacts. Usage is recalculated every 6-12 hours.

That is an account storage condition, not a broken build, and it was marking the whole run
red. The artifact is a convenience, not a gate, so the step is now continue-on-error.

The underlying quota still needs clearing (Settings -> Billing -> Storage, or let old
artifacts age out) for coverage reports to reappear.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -201,6 +201,12 @@ jobs:
       - name: Upload coverage
         uses: actions/upload-artifact@v4
         if: always()
+        # Non-blocking: this is a convenience artifact, not a gate. When the account's
+        # artifact storage quota is full the upload fails with
+        # "Failed to CreateArtifact: Artifact storage quota has been hit" and marked the
+        # whole run red even though every test step passed. A quota problem is not a
+        # broken build.
+        continue-on-error: true
         with:
           name: coverage-report
           path: coverage.xml