Previously, `image_caption_enabled_validator_spec` saved the
`ai_image_caption_agent` override with a string key via
`SiteSetting.provider.save`, so the symbol-keyed `remove_override!`
cleanup never cleared it and a rolled-back agent id leaked into later
specs in the same worker, making `generate_post_image_captions_spec`
fail with `agent_missing` whenever the two ran together.
This change saves the override with a symbol key so it round-trips with
the standard cleanup, keeping the setting isolated per example.