### What problem does this PR solve?
This PR adds non-streaming chat support for the Jina Go model provider.
The Jina provider was added with embedding, rerank, model listing, and
connection checking, but `ChatWithMessages` still returned a
not-implemented error even though Jina exposes an OpenAI-compatible
`/v1/chat/completions` endpoint.
Closes#14933
**The following functionalities are now supported:**
### **Jina:**
- [x] Chat
- [ ] Stream Chat
- [x] Embedding
- [x] Rerank
- [x] Model listing
- [x] Provider connection checking
- [ ] Balance
### **Implementation details:**
- Implements `JinaModel.ChatWithMessages`
- Sends `Authorization: Bearer <api-key>` and JSON chat completion
requests
- Validates API key, model name, messages, and configured region before
making requests
- Forwards supported chat config fields: `max_tokens`, `temperature`,
`top_p`, and `stop`
- Parses the first chat completion choice into `ChatResponse.Answer`
- Adds `jina-ai/jina-vlm` as a chat-capable model in
`conf/models/jina.json`
- Adds focused unit tests for request construction, auth, response
parsing, validation errors, provider errors, and region handling
**Verification:**
```plaintext
docker run --rm -v $PWD:/repo -w /repo golang:1.25 sh -c '/usr/local/go/bin/gofmt -w internal/entity/models/jina.go internal/entity/models/jina_test.go && /usr/local/go/bin/go test -vet=off ./internal/entity/models -run TestJina -count=1'
ok ragflow/internal/entity/models 0.037s
```
Note: `go test ./internal/entity/models -run TestJina -count=1`
currently hits unrelated existing vet findings in other provider files,
so the focused Jina tests were run with `-vet=off`.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
---------
Co-authored-by: Jin Hai <haijin.chn@gmail.com>
