fix(bq-materialize): code-review follow-ups for d8a22996

- tests/test_bq_cost_guardrail.py: assert fail-open warning is logged
  (test previously only proved fail-open doesn't crash; review note:
  warning is the only operator-visible signal of the silent failure).
- extractor._wrap_admin_sql_for_jobs_api: docstring no longer claims
  DuckDB-flavor SQL is rejected — the function performs no inner-SQL
  validation; the v24 migration + register-time validator are the
  real enforcement points.
- extractor.materialize_query: safe_path uses _escape_sql_string_literal
  instead of inline replace, for one-place-to-update consistency.
- extractor: import hashlib hoisted to module-level imports.

connectors/bigquery/extractor.py

@@ -3,6 +3,7 @@
 No data is downloaded. All queries go directly to BigQuery via DuckDB extension ATTACH.
 """
 
+import hashlib
 import logging
 import os
 import re
@@ -89,10 +90,11 @@ def _wrap_admin_sql_for_jobs_api(billing_project: str, inner_sql: str) -> str:
             should fail closed not silently lose budget to the wrong
             project).
         inner_sql: BigQuery-flavor SQL the admin registered as
-            ``source_query``. Must use BQ syntax (backticks for dashed
-            identifiers, native function calls). DuckDB-flavor `bq."ds"."t"`
-            is NOT acceptable here — the v24 migration converts existing
-            rows; new registrations are validated upstream.
+            ``source_query``. Should be BigQuery-native; DuckDB-flavor
+            `bq."ds"."t"` references are not enforced here but will fail at
+            COPY time inside the BQ jobs API. Existing rows are converted by
+            the v24 schema migration; new rows are validated upstream at
+            register/PUT.
 
     Returns:
         A DuckDB-parseable SQL fragment suitable as the operand of
@@ -471,7 +473,7 @@ def materialize_query(
             )
 
         try:
-            safe_path = str(tmp_path).replace("'", "''")
+            safe_path = _escape_sql_string_literal(str(tmp_path))
             conn.execute(
                 f"COPY ({wrapped_sql}) TO '{safe_path}' (FORMAT PARQUET)"
             )
@@ -489,7 +491,6 @@ def materialize_query(
     # thread — a 10 GiB parquet means 50+ seconds of disk I/O blocking other
     # requests. Hashing here keeps the open-file handle hot from the COPY
     # round and removes the second read. Devil's-advocate review item.
-    import hashlib
     h = hashlib.md5()
     with open(tmp_path, "rb") as f:
         for chunk in iter(lambda: f.read(8192), b""):

tests/test_bq_cost_guardrail.py

@@ -128,22 +128,26 @@ def test_zero_max_bytes_skips_dry_run(tmp_path):
     assert stats["rows"] == 1
 
 
-def test_dry_run_failure_is_fail_open(tmp_path):
+def test_dry_run_failure_is_fail_open(tmp_path, caplog):
     """If the dry-run errors (DuckDB syntax, missing google lib, transient
     upstream failure) we don't block — log + proceed with COPY. Operators
     who need hard-fail watch logs for the warning."""
+    import logging
+
     out = tmp_path / "extracts" / "bigquery"
     out.mkdir(parents=True)
     bq = _bq_with_seed({"bq.test.tiny": "SELECT 1 AS n"})
 
-    with patch(
-        "app.api.v2_scan._bq_dry_run_bytes", side_effect=RuntimeError("boom")
-    ):
-        stats = materialize_query(
-            table_id="t1",
-            sql="SELECT * FROM bq.test.tiny",
-            bq=bq,
-            output_dir=str(out),
-            max_bytes=10 * 2**30,
-        )
+    with caplog.at_level(logging.WARNING, logger="connectors.bigquery.extractor"):
+        with patch(
+            "app.api.v2_scan._bq_dry_run_bytes", side_effect=RuntimeError("boom")
+        ):
+            stats = materialize_query(
+                table_id="t1",
+                sql="SELECT * FROM bq.test.tiny",
+                bq=bq,
+                output_dir=str(out),
+                max_bytes=10 * 2**30,
+            )
     assert stats["rows"] == 1
+    assert "fail-open" in caplog.text