fix: BQ COUNT subquery alias, wrap ImportError in RemoteQueryError

- Add AS _cnt alias to COUNT(*) subquery (BQ Standard SQL requires it)
- Catch ImportError in _get_bq_client() and raise RemoteQueryError
  so API endpoint returns proper 400 instead of 500

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/remote_query.py

@@ -267,7 +267,7 @@ class RemoteQueryEngine:
         client = self._get_bq_client()
 
         # --- Phase 1a: COUNT(*) pre-check ---
-        count_sql = f"SELECT COUNT(*) FROM ({bq_sql})"
+        count_sql = f"SELECT COUNT(*) FROM ({bq_sql}) AS _cnt"
         try:
             count_job = client.query(count_sql)
             count_arrow = count_job.to_arrow()
@@ -413,9 +413,14 @@ class RemoteQueryEngine:
             project = os.environ.get("BIGQUERY_PROJECT", "unknown")
             return self._bq_client_factory(project)
 
-        # Trigger ImportError early if the package is missing.
-        # This is a lazy import so the module stays usable without BQ installed.
-        import google.cloud.bigquery as _bq_module  # noqa: PLC0415, F401
+        # Lazy import so the module stays usable without BQ installed.
+        try:
+            import google.cloud.bigquery as _bq_module  # noqa: PLC0415, F401
+        except ImportError:
+            raise RemoteQueryError(
+                "google-cloud-bigquery is not installed. Install with: pip install google-cloud-bigquery",
+                error_type="bq_error",
+            )
 
         project = os.environ.get("BIGQUERY_PROJECT")
         if not project:

tests/test_remote_query.py

@@ -124,7 +124,7 @@ class TestRemoteQueryEngineRegister:
             pass  # Expected — no BQ package in test env
 
     def test_register_bq_missing_package(self, analytics_conn):
-        """When google-cloud-bigquery is not installed, engine must raise ImportError."""
+        """When google-cloud-bigquery is not installed, engine must raise RemoteQueryError."""
         engine = RemoteQueryEngine(
             analytics_conn,
             # No factory — will try to import google.cloud.bigquery
@@ -133,7 +133,7 @@ class TestRemoteQueryEngineRegister:
         )
 
         with patch.dict(sys.modules, {"google": None, "google.cloud": None, "google.cloud.bigquery": None}):
-            with pytest.raises((ImportError, ModuleNotFoundError)):
+            with pytest.raises(RemoteQueryError, match="google-cloud-bigquery"):
                 engine.register_bq("bq_alias", "SELECT 1")