[MISC] Propagate line-item executor usage_records up the prompt chain (#1970)

LINE_ITEM prompts dispatch to the cloud line_item_extractor executor
via `_run_line_item_extraction`, but the legacy executor's
`_execute_single_prompt` returned `[]` instead of the executor's
flushed usage records. The records existed in
`line_item_result.metadata["usage_records"]` and were dropped on the
floor, so the structure_pipeline / answer_prompt outer
ExecutionResult never carried them to executor/tasks.py — and the
token_usage table stayed empty for line-item prompts.

Drain `line_item_result.metadata.usage_records`,
return them from `_run_line_item_extraction`, and forward through
the LINE_ITEM branch of `_execute_single_prompt` so they ride along
with the per-prompt usage accumulator like every other prompt type.

Paired with the cloud-side fix in unstract-cloud#1498 which adds the
flush at the line_item / agentic_table / etc. executor side.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: chandrasekharan-zipstack

workers/executor/executors/legacy_executor.py

@@ -1720,7 +1720,7 @@ def _execute_single_prompt(
             return []
 
         if output.get(PSKeys.TYPE) == PSKeys.LINE_ITEM:
-            self._run_line_item_extraction(
+            return self._run_line_item_extraction(
                 output=output,
                 context=context,
                 structured_output=structured_output,
@@ -1739,7 +1739,6 @@ def _execute_single_prompt(
                 },
                 shim=shim,
             )
-            return []
 
         usage_kwargs = {"run_id": run_id, "execution_id": execution_id}
         llm, embedding, vector_db = self._init_llm_and_retrieval(
@@ -2070,7 +2069,7 @@ def _run_line_item_extraction(
         metrics: dict[str, Any],
         prompt_run_args: dict[str, Any],
         shim: Any,
-    ) -> None:
+    ) -> list[dict[str, Any]]:
         """Delegate LINE_ITEM prompt to the line_item executor plugin.
 
         ``prompt_run_args`` bundles the per-prompt scalars passed from
@@ -2118,6 +2117,10 @@ def _run_line_item_extraction(
         shim.stream_log(f"Running line-item extraction for: `{prompt_name}`")
         line_item_result = line_item_executor.execute(line_item_ctx)
 
+        usage_records: list[dict[str, Any]] = list(
+            (line_item_result.metadata or {}).get("usage_records") or []
+        )
+
         if line_item_result.success:
             data = line_item_result.data or {}
             structured_output[prompt_name] = data.get("output", "")
@@ -2143,6 +2146,7 @@ def _run_line_item_extraction(
                 f"Line-item extraction failed for `{prompt_name}`: {error_msg}",
                 level=LogLevel.ERROR,
             )
+        return usage_records
 
     @staticmethod
     def _apply_type_conversion(