fix(antigravity): refresh default user agent version

The Antigravity service fingerprints clients by the version embedded in the User-Agent header. The built-in default (1.23.2) had drifted far behind current real client builds, so requests using the default UA started failing OAuth and probe flows.

Bump DefaultUserAgentVersion from 1.23.2 to 2.9.1 (current client build) and update the constant test accordingly. The default remains overridable via ANTIGRAVITY_USER_AGENT_VERSION and the admin setting.

backend/internal/pkg/antigravity/oauth.go

@@ -34,7 +34,7 @@ const (
 	AntigravityUserAgentVersionEnv = "ANTIGRAVITY_USER_AGENT_VERSION"
 
 	// DefaultUserAgentVersion 是未通过环境变量或后台设置覆盖时使用的默认版本号。
-	DefaultUserAgentVersion = "1.23.2"
+	DefaultUserAgentVersion = "2.9.1"
 
 	// 固定的 redirect_uri（用户需手动复制 code）
 	RedirectURI = "http://localhost:8085/callback"

backend/internal/pkg/antigravity/oauth_test.go

@@ -693,7 +693,7 @@ func TestConstants_值正确(t *testing.T) {
 	if RedirectURI != "http://localhost:8085/callback" {
 		t.Errorf("RedirectURI 不匹配: got %s", RedirectURI)
 	}
-	if GetUserAgent() != "antigravity/1.23.2 windows/amd64" {
+	if GetUserAgent() != "antigravity/2.9.1 windows/amd64" {
 		t.Errorf("UserAgent 不匹配: got %s", GetUserAgent())
 	}
 	if SessionTTL != 30*time.Minute {