ModelID, when set, instructs the handler to substitute any missing
secrets (APIKey, AppSecret via ExtraConfig) from the stored model
record before assembling the test client. This lets the "Test
connection" button work on existing models without making the
frontend reload — and ship — the plaintext API key. Other fields
(BaseURL, ModelName, etc.) on this request still override the
stored values, so a user can validate a new endpoint against the
existing credentials in one click.