fix(models): make PendingSubmission.app_name optional for backward compat

Older pending records (created before app_name was required) may not
have the field. If such a record exists in the date-sharded pending
store, any operation that loads the store fails Pydantic validation and
blocks prepare/list/get/cancel.

Make app_name optional with default None on the persistence model while
keeping it required in PrepareSubmitJobRequest, so new submissions still
must provide it but old records can be loaded and cancelled.

spark_executor/models.py

@@ -86,7 +86,7 @@ class Connection(BaseModel):
 
 class PendingSubmission(BaseModel):
     pending_id: str
-    app_name: str
+    app_name: str | None = None
 
     connection: str
     master: str