Function validate_semantic_option 

pub fn validate_semantic_option(key: &str, value: &str) -> bool

Validates a greptime.semantic.* option’s value against its allowed domain.

Open-value keys (unit, original_name, pipeline, conventions) accept any non-empty string. Closed-domain keys accept a fixed set, plus the unknown sentinel, plus mixed for the keys where one long-lived table can legitimately see multiple values. Keys not in SEMANTIC_OPTION_KEYS are rejected.