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/logredact/redact_test.go

@@ -0,0 +1,39 @@
+package logredact
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestRedactText_JSONLike(t *testing.T) {
+	in := `{"access_token":"ya29.a0AfH6SMDUMMY","refresh_token":"1//0gDUMMY","other":"ok"}`
+	out := RedactText(in)
+	if out == in {
+		t.Fatalf("expected redaction, got unchanged")
+	}
+	if want := `"access_token":"***"`; !strings.Contains(out, want) {
+		t.Fatalf("expected %q in %q", want, out)
+	}
+	if want := `"refresh_token":"***"`; !strings.Contains(out, want) {
+		t.Fatalf("expected %q in %q", want, out)
+	}
+}
+
+func TestRedactText_QueryLike(t *testing.T) {
+	in := "access_token=ya29.a0AfH6SMDUMMY refresh_token=1//0gDUMMY"
+	out := RedactText(in)
+	if strings.Contains(out, "ya29") || strings.Contains(out, "1//0") {
+		t.Fatalf("expected tokens redacted, got %q", out)
+	}
+}
+
+func TestRedactText_GOCSPX(t *testing.T) {
+	in := "client_secret=GOCSPX-abcdefghijklmnopqrstuvwxyz_0123456789"
+	out := RedactText(in)
+	if strings.Contains(out, "abcdefghijklmnopqrstuvwxyz") {
+		t.Fatalf("expected secret redacted, got %q", out)
+	}
+	if !strings.Contains(out, "client_secret=***") {
+		t.Fatalf("expected key redacted, got %q", out)
+	}
+}