feat(providers): add extra_body config to inject custom fields into request body

Allow configuring provider-specific fields like reasoning_split for minimax via
the model config's extra_body map. These fields are merged into the request
body last, giving them precedence over default values.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/backend/api/models.go

@@ -31,12 +31,13 @@ type modelResponse struct {
 	Proxy      string `json:"proxy,omitempty"`
 	AuthMethod string `json:"auth_method,omitempty"`
 	// Advanced fields
-	ConnectMode    string `json:"connect_mode,omitempty"`
-	Workspace      string `json:"workspace,omitempty"`
-	RPM            int    `json:"rpm,omitempty"`
-	MaxTokensField string `json:"max_tokens_field,omitempty"`
-	RequestTimeout int    `json:"request_timeout,omitempty"`
-	ThinkingLevel  string `json:"thinking_level,omitempty"`
+	ConnectMode    string         `json:"connect_mode,omitempty"`
+	Workspace      string         `json:"workspace,omitempty"`
+	RPM            int            `json:"rpm,omitempty"`
+	MaxTokensField string         `json:"max_tokens_field,omitempty"`
+	RequestTimeout int            `json:"request_timeout,omitempty"`
+	ThinkingLevel  string         `json:"thinking_level,omitempty"`
+	ExtraBody      map[string]any `json:"extra_body,omitempty"`
 	// Meta
 	Configured bool `json:"configured"`
 	IsDefault  bool `json:"is_default"`
@@ -81,6 +82,7 @@ func (h *Handler) handleListModels(w http.ResponseWriter, r *http.Request) {
 			MaxTokensField: m.MaxTokensField,
 			RequestTimeout: m.RequestTimeout,
 			ThinkingLevel:  m.ThinkingLevel,
+			ExtraBody:      m.ExtraBody,
 			Configured:     configured[i],
 			IsDefault:      m.ModelName == defaultModel,
 		})
@@ -183,6 +185,9 @@ func (h *Handler) handleUpdateModel(w http.ResponseWriter, r *http.Request) {
 	if mc.APIKey() == "" {
 		mc.SetAPIKey(cfg.ModelList[idx].APIKey())
 	}
+	if mc.ExtraBody == nil {
+		mc.ExtraBody = cfg.ModelList[idx].ExtraBody
+	}
 
 	cfg.ModelList[idx] = &mc