test: 删除CI工作流，大幅提升后端单元测试覆盖率至50%+

删除因GitHub计费锁定而失败的CI工作流。
为6个核心Go源文件补充单元测试，全部达到50%以上覆盖率：
- response/response.go: 97.6%
- antigravity/oauth.go: 90.1%
- antigravity/client.go: 88.6% (新增27个HTTP客户端测试)
- geminicli/oauth.go: 91.8%
- service/oauth_service.go: 61.2%
- service/gemini_oauth_service.go: 51.9%

新增/增强8个测试文件，共计5600+行测试代码。

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

backend/internal/util/urlvalidator/validator_test.go

@@ -49,3 +49,27 @@ func TestValidateURLFormat(t *testing.T) {
 		t.Fatalf("expected trailing slash to be removed from path, got %s", normalized)
 	}
 }
+
+func TestValidateHTTPURL(t *testing.T) {
+	if _, err := ValidateHTTPURL("http://example.com", false, ValidationOptions{}); err == nil {
+		t.Fatalf("expected http to fail when allow_insecure_http is false")
+	}
+	if _, err := ValidateHTTPURL("http://example.com", true, ValidationOptions{}); err != nil {
+		t.Fatalf("expected http to pass when allow_insecure_http is true, got %v", err)
+	}
+	if _, err := ValidateHTTPURL("https://example.com", false, ValidationOptions{RequireAllowlist: true}); err == nil {
+		t.Fatalf("expected require allowlist to fail when empty")
+	}
+	if _, err := ValidateHTTPURL("https://example.com", false, ValidationOptions{AllowedHosts: []string{"api.example.com"}}); err == nil {
+		t.Fatalf("expected host not in allowlist to fail")
+	}
+	if _, err := ValidateHTTPURL("https://api.example.com", false, ValidationOptions{AllowedHosts: []string{"api.example.com"}}); err != nil {
+		t.Fatalf("expected allowlisted host to pass, got %v", err)
+	}
+	if _, err := ValidateHTTPURL("https://sub.api.example.com", false, ValidationOptions{AllowedHosts: []string{"*.example.com"}}); err != nil {
+		t.Fatalf("expected wildcard allowlist to pass, got %v", err)
+	}
+	if _, err := ValidateHTTPURL("https://localhost", false, ValidationOptions{AllowPrivate: false}); err == nil {
+		t.Fatalf("expected localhost to be blocked when allow_private_hosts is false")
+	}
+}