Validate Schedule Configuration

Validate raw combined configuration JSON without converting errors to HTTP failures.

Args: payload: Untrusted JSON object containing scheduler and time-slot sections.

Returns: Structured validity, ordered diagnostics, and a fingerprint when valid.

Raises: None for configuration errors; they are encoded in the returned diagnostics.

Behavior: Validation uses the same schema and combined cross-reference rules as submission, but preserves all discovered configuration problems as response records. It does not create a scheduler, consume session capacity, or invoke Z3, making the endpoint suitable for pre-submission feedback.