ci(coverage): harden coverage guard step

- Add explicit set -eo pipefail so npm test:coverage exit code
  propagates through the pipe (not just tee's always-0 exit)
- Scope log file to github.run_id to prevent stale-log false positives
  on retried steps sharing the same runner /tmp
- Tighten grep pattern to \[birpc\] rpc is closed to avoid matching
  unrelated log lines that happen to contain "rpc is closed"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -44,7 +44,9 @@ jobs:
 
       - name: Run coverage (server + client)
         shell: bash
-        run: npm run test:coverage 2>&1 | tee /tmp/coverage-test.log
+        run: |
+          set -eo pipefail
+          npm run test:coverage 2>&1 | tee /tmp/coverage-test-${{ github.run_id }}.log
         working-directory: frontend
         env:
           TZ: Europe/Berlin
@@ -53,9 +55,9 @@ jobs:
         shell: bash
         if: always()
         run: |
-          if grep -q "rpc is closed" /tmp/coverage-test.log 2>/dev/null; then
+          if grep -q "\[birpc\] rpc is closed" /tmp/coverage-test-${{ github.run_id }}.log 2>/dev/null; then
             echo "FAIL: [birpc] rpc is closed teardown race detected in coverage run"
-            grep "rpc is closed" /tmp/coverage-test.log
+            grep "\[birpc\] rpc is closed" /tmp/coverage-test-${{ github.run_id }}.log
             exit 1
           fi