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_test.go

@@ -0,0 +1,70 @@
+package modelchannel
+
+import "testing"
+
+func TestNormalizeOpenAIEndpoint(t *testing.T) {
+	cases := []struct {
+		name         string
+		providerType string
+		endpoint     string
+		want         string
+	}{
+		// 非 openai 类型始终返回空
+		{"anthropic_ignored", "anthropic", "/v1/responses", ""},
+		{"empty_type_ignored", "", "/v1/responses", ""},
+
+		// 预设值
+		{"empty_defaults_to_responses", "openai", "", OpenAIEndpointResponses},
+		{"preset_responses", "openai", "/v1/responses", OpenAIEndpointResponses},
+		{"preset_chat_completions", "openai", "/v1/chat/completions", OpenAIEndpointChatCompletions},
+		{"preset_custom", "openai", "/custom", OpenAIEndpointCustom},
+
+		// 非法值：不再允许任意自定义路径，只接受三个预设值
+		{"arbitrary_path_rejected", "openai", "/v4/chat/completions", ""},
+		{"arbitrary_path_rejected_2", "openai", "/chat/completions", ""},
+		{"missing_slash", "openai", "v1/responses", ""},
+		{"slash_only", "openai", "/", ""},
+		{"whitespace", "openai", "   ", OpenAIEndpointResponses},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := NormalizeOpenAIEndpoint(tc.providerType, tc.endpoint)
+			if got != tc.want {
+				t.Fatalf("NormalizeOpenAIEndpoint(%q, %q) = %q, want %q", tc.providerType, tc.endpoint, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestOpenAIEndpointShape(t *testing.T) {
+	cases := []struct {
+		endpoint string
+		want     string
+	}{
+		// Responses 形态
+		{"/v1/responses", "responses"},
+		{"/v4/responses", "responses"},
+		{"/responses", "responses"},
+
+		// Chat Completions 形态
+		{"/v1/chat/completions", "chat/completions"},
+		{"/v4/chat/completions", "chat/completions"},
+		{"/chat/completions", "chat/completions"},
+
+		// /custom 不含已知后缀 → 兜底走 chat/completions
+		{"/custom", "chat/completions"},
+
+		// 兜底默认
+		{"", "chat/completions"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.endpoint, func(t *testing.T) {
+			got := OpenAIEndpointShape(tc.endpoint)
+			if got != tc.want {
+				t.Fatalf("OpenAIEndpointShape(%q) = %q, want %q", tc.endpoint, got, tc.want)
+			}
+		})
+	}
+}