Function validate_index_prefix 

pub fn validate_index_prefix(prefix: &str) -> Result<(), String>

Check that prefix is a legal leading fragment of an OpenSearch index name.

An empty prefix is valid (the default — no prefix). A non-empty prefix must be lowercase, contain none of the OpenSearch-forbidden characters, and start with an ASCII letter or digit (an index name may not begin with _/-/+). Returns a human-readable reason on failure, suitable for surfacing at config resolution time.