feat(submit): update_pending_job tool for editing PENDING submissions

Add an update_pending_job MCP tool that lets callers modify parameters
of a PENDING submission before confirm_submit_job:
  - queue, executor_memory, executor_cores, num_executors
  - app_name
  - extra_args
  - script_path (re-validates file existence and SQL guard)

Only PENDING submissions can be updated; SUBMITTED/CANCELLED/FAILED are
rejected with HTTP 400.

spark_executor/server.py

@@ -27,6 +27,7 @@ from spark_executor.tools.requests import (
     ReadJobFileRequest,
     SaveConnectionRequest,
     UpdateJobFileRequest,
+    UpdatePendingJobRequest,
 )
 from spark_executor.tools.status import get_job_status
 from spark_executor.tools.submit import (
@@ -35,6 +36,7 @@ from spark_executor.tools.submit import (
     get_pending_job,
     list_pending_jobs,
     prepare_submit_job,
+    update_pending_job,
 )
 
 from spark_executor.tools.result import get_job_result
@@ -138,6 +140,19 @@ def _get_pending_job(req: PendingIdRequest):
     return get_pending_job(req.pending_id)
 
 
+@app.post(
+    "/update_pending_job",
+    summary="Update an unsubmitted pending submission",
+    description=(
+        "Modify parameters of a PENDING submission before confirm_submit_job. "
+        "Only the provided fields are changed. If script_path is changed, the "
+        "new file must exist and pass the SQL guard."
+    ),
+)
+def _update_pending_job(req: UpdatePendingJobRequest):
+    return update_pending_job(**req.model_dump(exclude_none=True))
+
+
 @app.post(
     "/cancel_pending_job",
     summary="Cancel a pending submission",