fix: detect SQL execution errors in _execute_task before passing to diff engine

When `execute_sql` fails, it returns `columns=['error']` with the error
message as a data row. Previously this was silently passed to the Rust
engine as data, causing confusing downstream failures. Now raises
`RuntimeError` immediately so the error propagates to the user.

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

Author: suryaiyer95

packages/altimate-engine/src/altimate_engine/sql/data_diff.py

@@ -55,6 +55,11 @@ def _execute_task(task: dict, warehouse: str) -> dict:
         SqlExecuteParams(sql=task["sql"], warehouse=warehouse, limit=100_000)
     )
 
+    # Detect error results: execute_sql returns columns=['error'] on failure
+    if result.columns and result.columns[0] == "error":
+        error_msg = result.rows[0][0] if result.rows else "Unknown SQL execution error"
+        raise RuntimeError(f"SQL execution failed for task {task['id']}: {error_msg}")
+
     # Convert SqlExecuteResult rows to the format expected by ReladiffSession.step()
     # Guard: executor returns a synthetic status row when row_count is 0 — skip it.
     rows: list[list[str | None]] = []