### What problem does this PR solve?
Adds the Go model provider driver for CometAPI, which is listed as
unchecked in the Go provider tracking issue #14736 and requested in
#14804. Without this, the Go layer falls back to the dummy driver for
the `cometapi` provider.
Fixes#14804
### What this PR includes
- New `internal/entity/models/cometapi.go` implementing `ModelDriver`
for CometAPI.
- New `conf/models/cometapi.json` with CometAPI base URLs and
representative chat / embedding models from the public catalog.
- `factory.go`: route `"cometapi"` to `NewCometAPIModel`.
- Unit tests in `internal/entity/models/cometapi_test.go`.
### Method coverage
- `ChatWithMessages`: `POST /v1/chat/completions`.
- `ChatStreamlyWithSender`: SSE streaming on the same endpoint.
- `Embed`: `POST /v1/embeddings`, including optional `dimensions`.
- `ListModels`: `GET /api/models` public catalog.
- `Balance`: `GET https://query.cometapi.com/user/quota?key=...`.
- `CheckConnection`: delegates to the quota query to verify the key.
- `Rerank`, ASR, TTS, OCR: return `no such method` for now.
No ModelDriver interface change. No new dependencies.
### How was this tested?
```bash
go test -vet=off -run TestCometAPI -count=1 ./internal/entity/models/...
go test -vet=off -count=1 ./internal/entity/models/...
```
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