ci(coverage): use grep -F for birpc guard to avoid BRE escaping

-F (fixed string) matches the literal pattern [birpc] rpc is closed
without relying on BRE bracket escaping, making the intent explicit
and immune to accidental regex interpretation.

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

.gitea/workflows/ci.yml

@@ -49,9 +49,9 @@ jobs:
         shell: bash
         if: always()
         run: |
-          if grep -q "\[birpc\] rpc is closed" /tmp/coverage-test-${{ github.run_id }}.log 2>/dev/null; then
+          if grep -qF "[birpc] rpc is closed" /tmp/coverage-test-${{ github.run_id }}.log 2>/dev/null; then
             echo "FAIL: [birpc] rpc is closed teardown race detected in coverage run"
-            grep "\[birpc\] rpc is closed" /tmp/coverage-test-${{ github.run_id }}.log
+            grep -F "[birpc] rpc is closed" /tmp/coverage-test-${{ github.run_id }}.log
             exit 1
           fi