refactor(auth): extract Antigravity OAuth constants to internal/auth

internal/auth/antigravity/constants.go

@@ -0,0 +1,34 @@
+// Package antigravity provides OAuth2 authentication functionality for the Antigravity provider.
+package antigravity
+
+// OAuth client credentials and configuration
+const (
+	ClientID     = "1071006060591-tmhssin2h21lcre235vtolojh4g403ep.apps.googleusercontent.com"
+	ClientSecret = "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"
+	CallbackPort = 51121
+)
+
+// Scopes defines the OAuth scopes required for Antigravity authentication
+var Scopes = []string{
+	"https://www.googleapis.com/auth/cloud-platform",
+	"https://www.googleapis.com/auth/userinfo.email",
+	"https://www.googleapis.com/auth/userinfo.profile",
+	"https://www.googleapis.com/auth/cclog",
+	"https://www.googleapis.com/auth/experimentsandconfigs",
+}
+
+// OAuth2 endpoints for Google authentication
+const (
+	TokenEndpoint    = "https://oauth2.googleapis.com/token"
+	AuthEndpoint     = "https://accounts.google.com/o/oauth2/v2/auth"
+	UserInfoEndpoint = "https://www.googleapis.com/oauth2/v1/userinfo?alt=json"
+)
+
+// Antigravity API configuration
+const (
+	APIEndpoint    = "https://cloudcode-pa.googleapis.com"
+	APIVersion     = "v1internal"
+	APIUserAgent   = "google-api-nodejs-client/9.15.1"
+	APIClient      = "google-cloud-sdk vscode_cloudshelleditor/0.1"
+	ClientMetadata = `{"ideType":"IDE_UNSPECIFIED","platform":"PLATFORM_UNSPECIFIED","pluginType":"GEMINI"}`
+)