Go: implement Embed (embeddings) in TogetherAI driver (#15017)

### What problem does this PR solve?

Fixes #15015

The TogetherAI Go driver in `internal/entity/models/togetherai.go`
shipped a stub `Embed` method that returned `"TogetherAI, no such
method"`, so TogetherAI could not be used as an embedding provider in
RAGFlow. This PR fills that gap.

TogetherAI exposes a public OpenAI-compatible embeddings endpoint at
`POST https://api.together.ai/v1/embeddings` that accepts the standard
`{model, input}` shape with `Authorization: Bearer <api_key>` (confirmed
in TogetherAI's official docs:
https://docs.together.ai/docs/embeddings-overview). Documented embedding
models include `intfloat/multilingual-e5-large-instruct`,
`BAAI/bge-large-en-v1.5`, and `BAAI/bge-base-en-v1.5`.

### Changes

- `internal/entity/models/togetherai.go`: implement
`TogetherAIModel.Embed`.
- Validate inputs (api key, model name) and short-circuit on empty
texts.
  - Resolve region with the existing `baseURLForRegion` helper.
  - Build URL from `URLSuffix.Embedding`.
- Send `{model, input}` POST body, add `dimensions` when
`embeddingConfig.Dimension > 0` (matches the pattern in #14735).
  - Bearer auth + JSON content type, mirroring the chat path.
- Parse `{data: [{embedding, index}]}` and reorder by `index`, rejecting
out-of-range indices, duplicates, and missing entries so the output
always lines up with the input. Same shape as the merged Mistral,
Upstage, and Novita Embed implementations.
- `conf/models/togetherai.json`:
  - Add `"embedding": "embeddings"` to `url_suffix`.
- Add default embedding model entries for
`intfloat/multilingual-e5-large-instruct`, `BAAI/bge-large-en-v1.5`, and
`BAAI/bge-base-en-v1.5`.

### Type of change

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

conf/models/togetherai.json

@@ -5,7 +5,8 @@
   },
   "url_suffix": {
     "chat": "chat/completions",
-    "models": "models"
+    "models": "models",
+    "embedding": "embeddings"
   },
   "class": "together",
   "models": [
@@ -29,6 +30,27 @@
       "model_types": [
         "chat"
       ]
+    },
+    {
+      "name": "intfloat/multilingual-e5-large-instruct",
+      "max_tokens": 514,
+      "model_types": [
+        "embedding"
+      ]
+    },
+    {
+      "name": "BAAI/bge-large-en-v1.5",
+      "max_tokens": 512,
+      "model_types": [
+        "embedding"
+      ]
+    },
+    {
+      "name": "BAAI/bge-base-en-v1.5",
+      "max_tokens": 512,
+      "model_types": [
+        "embedding"
+      ]
     }
   ]
 }

internal/entity/models/togetherai.go

@@ -385,8 +385,108 @@ func (t *TogetherAIModel) CheckConnection(apiConfig *APIConfig) error {
 	return err
 }
 
+type togetherAIEmbeddingData struct {
+	Embedding []float64 `json:"embedding"`
+	Object    string    `json:"object"`
+	Index     int       `json:"index"`
+}
+
+type togetherAIEmbeddingResponse struct {
+	Data   []togetherAIEmbeddingData `json:"data"`
+	Model  string                    `json:"model"`
+	Object string                    `json:"object"`
+}
+
 func (t *TogetherAIModel) Embed(modelName *string, texts []string, apiConfig *APIConfig, embeddingConfig *EmbeddingConfig) ([]EmbeddingData, error) {
-	return nil, fmt.Errorf("%s, no such method", t.Name())
+	if len(texts) == 0 {
+		return []EmbeddingData{}, nil
+	}
+
+	if apiConfig == nil || apiConfig.ApiKey == nil || *apiConfig.ApiKey == "" {
+		return nil, fmt.Errorf("api key is required")
+	}
+
+	if modelName == nil || strings.TrimSpace(*modelName) == "" {
+		return nil, fmt.Errorf("model name is required")
+	}
+
+	region := "default"
+	if apiConfig.Region != nil && *apiConfig.Region != "" {
+		region = *apiConfig.Region
+	}
+
+	baseURL, err := t.baseURLForRegion(region)
+	if err != nil {
+		return nil, err
+	}
+	url := fmt.Sprintf("%s/%s", baseURL, t.URLSuffix.Embedding)
+
+	reqBody := map[string]interface{}{
+		"model": *modelName,
+		"input": texts,
+	}
+	if embeddingConfig != nil && embeddingConfig.Dimension > 0 {
+		reqBody["dimensions"] = embeddingConfig.Dimension
+	}
+
+	jsonData, err := json.Marshal(reqBody)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal request: %w", err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), nonStreamCallTimeout)
+	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 := t.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("TogetherAI embeddings API error: %s, body: %s", resp.Status, string(body))
+	}
+
+	var parsed togetherAIEmbeddingResponse
+	if err = json.Unmarshal(body, &parsed); err != nil {
+		return nil, fmt.Errorf("failed to parse response: %w", err)
+	}
+
+	embeddings := make([]EmbeddingData, len(texts))
+	filled := make([]bool, len(texts))
+	for _, item := range parsed.Data {
+		if item.Index < 0 || item.Index >= len(texts) {
+			return nil, fmt.Errorf("togetherai: response index %d out of range for %d inputs", item.Index, len(texts))
+		}
+		if filled[item.Index] {
+			return nil, fmt.Errorf("togetherai: duplicate embedding index %d in response", item.Index)
+		}
+		embeddings[item.Index] = EmbeddingData{
+			Embedding: item.Embedding,
+			Index:     item.Index,
+		}
+		filled[item.Index] = true
+	}
+	for i, ok := range filled {
+		if !ok {
+			return nil, fmt.Errorf("togetherai: missing embedding for input index %d", i)
+		}
+	}
+
+	return embeddings, nil
 }
 
 func (t *TogetherAIModel) Rerank(modelName *string, query string, documents []string, apiConfig *APIConfig, rerankConfig *RerankConfig) (*RerankResponse, error) {