test: close coverage gaps and add frontend test infrastructure

Backend (516 tests, 94% coverage):
- Add azure_openai endpoint/deployment validation tests (config.py -> 100%)
- Add _total_conversations and _avg_turns direct tests (queries.py -> 100%)
- Add transformer edge cases: list content, string checkpoint, invalid JSON,
  malformed message graceful skip (transformer.py -> 93%)
- Add safety combined status_code+error_message interaction tests
- Fix ambiguous 200/422 assertion to strict 422
- Add E2E pagination shape assertions (total, page, per_page, row count)
- Fix ReplayPool mock to respect LIMIT/OFFSET params

Frontend (23 tests, vitest + happy-dom + @testing-library/react):
- Add vitest infrastructure with happy-dom environment
- Add api.ts tests: success, HTTP error, success=false, URL encoding
- Add DashboardPage tests: loading, data, error, empty states
- Add ReplayListPage tests: loading, empty, data, error, status badge classes
- Add ReplayPage tests: loading, steps, empty, error states

backend/tests/unit/test_safety.py

@@ -67,6 +67,17 @@ class TestClassifyMcpError:
     def test_unknown_message(self) -> None:
         assert classify_mcp_error(error_message="Something happened") == "unknown"
 
+    def test_status_code_takes_precedence_over_message(self) -> None:
+        # 429 is transient by code; message would classify as validation
+        assert classify_mcp_error(status_code=429, error_message="invalid param") == "transient"
+
+    def test_non_classified_status_falls_through_to_message(self) -> None:
+        # 200 is not in any status set, so message classification takes over
+        assert classify_mcp_error(status_code=200, error_message="timed out") == "transient"
+
+    def test_no_args_returns_unknown(self) -> None:
+        assert classify_mcp_error() == "unknown"
+
 
 class TestRetryPolicy:
     def test_transient_is_retryable(self) -> None: