Implement logic to resolve and inject Replicate Kokoro model-specific language
codes based on language tag and voice, using new mapping utilities. Update
`buildReplicateInput` to set `language_code` for the built-in Kokoro model.
Expand language utilities with code mapping and expose `resolveReplicateKokoroLanguageCode`.
Enhance tests to cover language code resolution and input mapping for Replicate
Kokoro, and verify language input key discovery for built-in models.
