Draft a Custom REST source manifest from API documentation using an LLM. Reads the docs (a URL fetched server-side, or pasted text / OpenAPI spec), asks the model to author a RESTAPIConfig manifest, and validates it against the create-path checks — repairing against validation errors up to a small budget. Returns the manifest for the user to review and tweak in the builder before creating the source; it does NOT create anything. Gated by the `dwh-custom-source-ai-builder` flag, and requires the org to have approved AI data processing, since the docs are sent to the LLM gateway.