Go: implement Rerank in Gitee AI driver (#14656)

### What problem does this PR solve?

The Gitee AI Go driver shipped with a stub \`Rerank\` method that
returned \`Rerank not implemented\`, even though
\`conf/models/gitee.json\` already wires the rerank URL suffix at
\`\"rerank\": \"rerank\"\`. The same config did not list any
rerank model, so the picker had nothing to select.

So a Gitee tenant could not use BAAI/bge-reranker-v2-m3 as a reranker
through the Go layer today, even though the
infrastructure was one config entry and one method body away.

### What this PR includes

- \`conf/models/gitee.json\`: add \`BAAI/bge-reranker-v2-m3\` to the
\`models\` array.
- \`internal/entity/models/gitee.go\`: replace the \`Rerank\` stub with
a real implementation. Adds two small local types
that match the OpenAI-compatible \`/rerank\` shape already used by the
SiliconFlow and ZhipuAI drivers.

No factory change. No interface change.

### How the driver works

- Validate \`apiConfig\` and the API key, validate the model name,
resolve the region with a default fallback, build the
  URL from \`BaseURL[region] + URLSuffix.Rerank\`.
- Use a per-call \`context.WithTimeout(30s)\` and
\`http.NewRequestWithContext\`, matching the pattern the
  recently merged Aliyun Encode and the OpenAI driver already use.
- Send \`{model, query, documents, top_n, return_documents:false}\` in
the body.
- Parse \`results[*].relevance_score\` and copy each score into the
output slice indexed by \`results[*].index\`, so the
output order matches the input order even if the API returns items in a
different order.
- Empty input returns \`[]float64{}\` with no HTTP call.
- An out-of-range result index returns a clear error rather than
silently skipping the entry.
- Non-200 responses propagate the upstream status line and body.

### Type of change

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

### How was this tested?

- \`go build ./internal/entity/models/...\` in a clean go 1.25 image
returns exit 0.
- The full method set on \`GiteeModel\` still matches the
\`ModelDriver\` interface.
- Pattern parity with the existing SiliconFlow Rerank and the recently
merged ZhipuAI Rerank (#14608).

Closes #14655

internal/entity/models/gitee.go

@@ -19,6 +19,7 @@ package models
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -402,9 +403,105 @@ func (z *GiteeModel) Encode(modelName *string, texts []string, apiConfig *APICon
 	return nil, fmt.Errorf("%s, no such method", z.Name())
 }
 
+type giteeRerankRequest struct {
+	Model           string   `json:"model"`
+	Query           string   `json:"query"`
+	Documents       []string `json:"documents"`
+	TopN            int      `json:"top_n"`
+	ReturnDocuments bool     `json:"return_documents"`
+}
+
+type giteeRerankResponse struct {
+	Results []struct {
+		Index          int     `json:"index"`
+		RelevanceScore float64 `json:"relevance_score"`
+	} `json:"results"`
+}
+
 // Rerank calculates similarity scores between query and texts
 func (z *GiteeModel) Rerank(modelName *string, query string, texts []string, apiConfig *APIConfig) ([]float64, error) {
-	return nil, fmt.Errorf("%s, Rerank not implemented", z.Name())
+	if len(texts) == 0 {
+		return []float64{}, nil
+	}
+
+	if apiConfig == nil || apiConfig.ApiKey == nil || *apiConfig.ApiKey == "" {
+		return nil, fmt.Errorf("api key is required")
+	}
+
+	if modelName == nil || *modelName == "" {
+		return nil, fmt.Errorf("model name is required")
+	}
+
+	region := "default"
+	if apiConfig.Region != nil && *apiConfig.Region != "" {
+		region = *apiConfig.Region
+	}
+
+	baseURL := z.BaseURL["default"]
+	if region != "default" {
+		if regional, ok := z.BaseURL[region]; ok && regional != "" {
+			baseURL = regional
+		}
+	}
+	if baseURL == "" {
+		return nil, fmt.Errorf("gitee: no base URL configured for default region")
+	}
+
+	url := fmt.Sprintf("%s/%s", strings.TrimSuffix(baseURL, "/"), z.URLSuffix.Rerank)
+
+	reqBody := giteeRerankRequest{
+		Model:           *modelName,
+		Query:           query,
+		Documents:       texts,
+		TopN:            len(texts),
+		ReturnDocuments: false,
+	}
+
+	jsonData, err := json.Marshal(reqBody)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal request: %w", err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, "POST", url, bytes.NewBuffer(jsonData))
+	if err != nil {
+		return nil, fmt.Errorf("failed to create request: %w", err)
+	}
+
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", fmt.Sprintf("Bearer %s", *apiConfig.ApiKey))
+
+	resp, err := z.httpClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("failed to send request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read response: %w", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("Gitee rerank API error: %s, body: %s", resp.Status, string(body))
+	}
+
+	var parsed giteeRerankResponse
+	if err = json.Unmarshal(body, &parsed); err != nil {
+		return nil, fmt.Errorf("failed to parse response: %w", err)
+	}
+
+	scores := make([]float64, len(texts))
+	for _, r := range parsed.Results {
+		if r.Index < 0 || r.Index >= len(texts) {
+			return nil, fmt.Errorf("unexpected rerank index %d for %d inputs", r.Index, len(texts))
+		}
+		scores[r.Index] = r.RelevanceScore
+	}
+
+	return scores, nil
 }
 
 func (z *GiteeModel) ListModels(apiConfig *APIConfig) ([]string, error) {