Update Search Tool

Update a stored search or fetch instance. Omitted fields are left as-is; an explicit null clears them.

api_key follows the same rule: omit it to keep the stored key, send a new one to rotate, or send null to clear it (a keyless SearXNG backend). The row is locked FOR UPDATE so the expected_updated_at check and the write it guards are atomic. The tool as it will be after the update is validated, so a change that would leave it unusable (clearing the key of a provider that needs one) is refused rather than stored. Its options are checked against the provider's when the update sets them or changes the provider, and fetch_tool when the update sets it, so rotating the key of an instance stored before those rules never trips on them. kind cannot change, so an instance never moves between the search and fetch maps.