fix(go-models): harden Hunyuan embedding validation (#15249)

## Summary
- Validate Hunyuan embedding model name and API key before building
requests.
- Reuse region-aware base URL validation for embedding requests.
- Replace the stale unsupported Embed test with happy-path and
validation coverage.

## What changed
- Added early Hunyuan Embed validation for missing model names and API
keys.
- Routed Embed through the same base URL region guard used by the other
Hunyuan methods.
- Updated Hunyuan tests to configure the embedding suffix and cover
Embed success plus invalid inputs.

## Why
Hunyuan Embed is implemented, but the existing test still expected it to
be unsupported and could panic before returning a normal validation
error. This keeps the implemented embedding path aligned with the
current driver behavior and prevents nil input panics.

Closes #15087
Refs #14736