feat: add support for custom OpenAI endpoint path

- Introduced a new endpoint option for OpenAI integrations, allowing users to specify a custom path.
- Updated relevant components and validation logic to accommodate the new endpoint.
- Enhanced documentation and error messages to reflect the addition of the custom endpoint option.

internal/modelchannel/identity.go

@@ -13,6 +13,7 @@ const ChannelIDHexLength = 16
 const (
 	OpenAIEndpointResponses       = "/v1/responses"
 	OpenAIEndpointChatCompletions = "/v1/chat/completions"
+	OpenAIEndpointCustom          = "/custom"
 )
 
 func NormalizeBaseURL(raw string) (string, error) {
@@ -39,20 +40,37 @@ func NormalizeBaseURL(raw string) (string, error) {
 	return normalized, nil
 }
 
+// NormalizeOpenAIEndpoint 归一化 OpenAI endpoint 路径。
+// 支持三个预设值：/v1/responses、/v1/chat/completions、/custom（自定义路径）。
+// 选 /custom 时，用户需在接口地址栏填写完整请求 URL。
 func NormalizeOpenAIEndpoint(providerType string, endpoint string) string {
 	if strings.TrimSpace(strings.ToLower(providerType)) != "openai" {
 		return ""
 	}
-	switch strings.ToLower(strings.TrimSpace(endpoint)) {
-	case "", OpenAIEndpointResponses:
+	normalized := strings.TrimSpace(endpoint)
+	switch normalized {
+	case "":
 		return OpenAIEndpointResponses
-	case OpenAIEndpointChatCompletions:
-		return OpenAIEndpointChatCompletions
+	case OpenAIEndpointResponses, OpenAIEndpointChatCompletions, OpenAIEndpointCustom:
+		return normalized
 	default:
 		return ""
 	}
 }
 
+// OpenAIEndpointShape 根据 endpoint 路径末段推断协议形态。
+// 返回 "responses"（Responses API）或 "chat/completions"（Chat Completions API）。
+// 这样 /v1/chat/completions、/v4/chat/completions、/chat/completions 都走同一协议分支。
+func OpenAIEndpointShape(endpoint string) string {
+	lower := strings.ToLower(strings.TrimSpace(endpoint))
+	switch {
+	case strings.HasSuffix(lower, "/responses"):
+		return "responses"
+	default:
+		return "chat/completions"
+	}
+}
+
 func BuildLegacyChannelID(baseURL string, modelID string, apiKey string, name string) string {
 	return buildChannelID([]string{
 		strings.TrimSpace(baseURL),