refactor: remove endpoint tests and enhance URL handling

- Deleted outdated test files for OpenAI endpoint functionality.
- Improved the OpenAIEndpointURL function to better handle custom paths and version segments.
- Introduced a new helper function, stripEndpointVersionPrefix, to streamline endpoint processing.

internal/modelchannel/identity_test.go

@@ -1,70 +0,0 @@
-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)
-			}
-		})
-	}
-}