[Data] Propagate branch in _append_and_commit(..) for iceberg overwrite save mode (#63922)

Fix bug where if a user passes `branch` as an `overwrite_kwarg` the
delete lands on staging but the subsequent append lands on main (the
default in `_append_and_commit`)

Now we should thread through this arg to the append phase as well.

Signed-off-by: Ayush Kumar <ayushk7102@gmail.com>

Author: ayushk7102

python/ray/data/_internal/datasource/iceberg_datasink.py

@@ -841,8 +841,9 @@ def _commit_overwrite(
             **self._overwrite_kwargs,
         )
 
-        # Append new data files and commit
-        self._append_and_commit(txn, data_files)
+        # Append on the same branch the delete targeted (defaults to "main").
+        branch = self._overwrite_kwargs.get("branch", "main")
+        self._append_and_commit(txn, data_files, branch=branch)
 
     def on_write_complete(self, write_result: WriteResult) -> None:
         """