fix: remove invalid expunge call on Pydantic FileModel (#20931)

fix: remove invalid expunge call on Pydantic FileModel
Files.get_file_by_id() returns a Pydantic FileModel, not an SQLAlchemy
ORM object. Calling db.expunge() on a Pydantic model fails with
UnmappedInstanceError since it lacks _sa_instance_state.
The expunge was also unnecessary because subsequent DB updates already
use fresh sessions via get_db() context manager.
Fixes #20925

backend/open_webui/routers/retrieval.py

@@ -1771,13 +1771,12 @@ def process_file(
                 }
             else:
                 try:
-                    # Release the database connection relative to the 'file' object
+                    # Commit any pending changes before the slow embedding step.
-                    # to prevent holding the connection during the slow embedding step.
+                    # Note: file is already a Pydantic model (not ORM), so no expunge needed.
-                    db.expunge(file)
                     db.commit()
 
                     # External embedding API takes time (5-60s+).
-                    # No DB connection is held here.
+                    # Subsequent updates use fresh sessions via get_db().
                     result = save_docs_to_vector_db(
                         request,
                         docs=docs,