## What
- Add TogetherAI as a chat provider backed by its OpenAI-compatible
`/v1/chat/completions` API
- Register TogetherAI in the Go model factory and provider config
- Support non-streaming chat, SSE streaming chat, model listing, and
connection checks
## Notes
- Uses the current TogetherAI OpenAI-compatible base URL
`https://api.together.ai/v1`
- Forwards documented chat parameters from `ChatConfig`: `max_tokens`,
`temperature`, `top_p`, `stop`, and GPT-OSS `reasoning_effort`
- Routes Together reasoning traces from `reasoning` /
`reasoning_content` into `ReasonContent`
## Tests
- `go test -vet=off -run TestTogetherAI -count=1
./internal/entity/models`
- `go test -vet=off -count=1 ./internal/entity/models`
Refs #14736