feat: add GiteeAI provider support to Go API server (#15131)

### What problem does this PR solve?

Closes #15090.

Adds GiteeAI support to the Go model-provider layer so GiteeAI chat
models can be routed through the Go API server using the same
OpenAI-compatible chat, streaming, model listing, and connection-check
flow used by other SaaS providers.

GiteeAI is implemented as a separate provider from the existing `gitee`
provider.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

## Summary

- Added a GiteeAI Go model driver.
- Added the GiteeAI provider catalog with default base URL
`https://ai.gitee.com/v1`.
- Registered `giteeai` in the model factory separately from `gitee`.
- Added focused provider tests for sync chat, streaming chat, model
listing, connection checks, base URL override, SSE parsing, `[DONE]`
handling, and unsupported methods.

## What changed

- Implemented `ChatWithMessages` for `POST /chat/completions`.
- Implemented `ChatStreamlyWithSender` with SSE parsing, `delta`
extraction, `finish_reason`, `[DONE]`, and `<think>` tag handling.
- Implemented `ListModels` for `GET /models`.
- Implemented `CheckConnection` by delegating to `ListModels`.
- Returned standard `no such method` errors for unsupported embedding,
rerank, image-to-text, ASR, and TTS paths.

## Tests

```bash
go test -vet=off ./internal/entity/models -run 'TestGiteeAI' -count=1
go test -vet=off ./internal/entity -run 'Test.*Provider|Test.*Model' -count=1
```

---------

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

conf/models/gitee.json

@@ -1,7 +1,9 @@
 {
   "name": "Gitee",
   "url": {
-    "default": "https://api.moark.com/v1"
+    "default": "https://api.moark.ai/v1",
+    "china": "https://api.moark.com/v1",
+    "deprecated": "https://ai.gitee.com/v1"
   },
   "url_suffix": {
     "chat": "chat/completions",