fix(test): assert _add_team_member_budget_table returns a copy, not mutation

PR #36234 refactored _add_team_member_budget_table to return
model_copy(update=...) instead of mutating its input, but the success
test still asserted result == team_info_response, which now fails
because the input stays unmutated with team_member_budget_table=None
while the returned copy carries the budget record. Rewrite the
assertions to pin the new no-mutation contract: the return value is a
distinct object, the input's budget field is untouched, the copy has
the budget attached, and the other fields still match.

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

tests/test_litellm/proxy/management_endpoints/test_team_endpoints.py

@@ -924,18 +924,18 @@ async def test_add_team_member_budget_table_success():
         team_id="test-team-123", team_alias="Test Team"
     )
 
-    # Call the function
     result = await _add_team_member_budget_table(
         team_member_budget_id="budget-123",
         prisma_client=mock_prisma_client,
         team_info_response_object=team_info_response,
     )
 
-    # Verify the result
-    assert result == team_info_response
+    assert result is not team_info_response
+    assert team_info_response.team_member_budget_table is None
     assert result.team_member_budget_table == mock_budget_record
+    assert result.team_id == team_info_response.team_id
+    assert result.team_alias == team_info_response.team_alias
 
-    # Verify database call was made correctly
     mock_prisma_client.db.litellm_budgettable.find_unique.assert_called_once_with(
         where={"budget_id": "budget-123"}
     )