test(vertex passthrough): surface unparseable spend log startTime

A row that matches the model and spend > 0 but whose startTime can't be parsed
was silently skipped, so a future /spend/logs format change would time out with
the "cost-tracking write was lost" message instead of pointing at the timestamp.
Print the offending value when that happens; matching behaviour is unchanged

tests/pass_through_tests/test_vertex_ai.py

@@ -163,7 +163,13 @@ async def get_request_spend_log(
             if float(row.get("spend") or 0.0) <= 0:
                 continue
             row_start = _parse_spend_log_start_time(row.get("startTime"))
-            if row_start is not None and row_start >= after_time:
+            if row_start is None:
+                print(
+                    f"matching row has unparseable startTime {row.get('startTime')!r}",
+                    row,
+                )
+                continue
+            if row_start >= after_time:
                 print(f"found spend log (elapsed={elapsed}s)", row)
                 return row
     return None