test(proxy): update team budget helper test for non-mutating return

The _add_team_member_budget_table helper was changed in #36234 to return
a model_copy of its input rather than mutating in place. The existing
test still asserted result == team_info_response (Pydantic structural
equality), which now fails because the returned copy has the budget
table set while the original does not.

Assert the preserved fields plus the new budget attachment, and add a
regression guard that the input response object is not mutated.

Co-authored-by: Krrish Dholakia <krrish-berri-2@users.noreply.github.com>

tests/test_litellm/proxy/management_endpoints/test_team_endpoints.py

@@ -931,9 +931,10 @@ async def test_add_team_member_budget_table_success():
         team_info_response_object=team_info_response,
     )
 
-    # Verify the result
-    assert result == team_info_response
+    assert result.team_id == team_info_response.team_id
+    assert result.team_alias == team_info_response.team_alias
     assert result.team_member_budget_table == mock_budget_record
+    assert team_info_response.team_member_budget_table is None
 
     # Verify database call was made correctly
     mock_prisma_client.db.litellm_budgettable.find_unique.assert_called_once_with(