feat(antigravity): initial steps for Antigravity anti-ban alignment

Cherry-picked from decolua/9router#141 (author: LinearSakana / zx <me@char.moe>)

- Implement client identity spoofing with numeric enums (ideType: 9, pluginType: 2)
- Add runtime platform detection for User-Agent and metadata
- Implement per-connection session ID caching (binary-compatible format)
- Add ANTIGRAVITY_HEADERS (X-Client-Name, X-Client-Version, x-goog-api-client)
- Add X-Machine-Session-Id header injection
- Align metadata/mode parameters across all Antigravity API calls
- Implement double injection for system prompt (raw + [ignore] wrapped)
- Rename internal anti-loop header to x-request-source for anonymity

Skipped: commit 6 (signature side-channel caching) — kept DEFAULT_THINKING_GEMINI_SIGNATURE
Co-authored-by: Cursor <cursoragent@cursor.com>

src/lib/oauth/constants/oauth.js

@@ -1,6 +1,20 @@
 /**
  * OAuth Configuration Constants
  */
+import { platform, arch } from "os";
+
+/**
+ * Get the platform enum value based on the current OS.
+ * Matches Antigravity binary's ClientMetadata.Platform enum.
+ */
+function getOAuthPlatformEnum() {
+  const os = platform();
+  const architecture = arch();
+  if (os === "darwin") return architecture === "arm64" ? 2 : 1;
+  if (os === "linux") return architecture === "arm64" ? 4 : 3;
+  if (os === "win32") return 5;
+  return 0;
+}
 
 // Claude OAuth Configuration (Authorization Code Flow with PKCE)
 export const CLAUDE_CONFIG = {
@@ -83,9 +97,17 @@ export const ANTIGRAVITY_CONFIG = {
   onboardUserEndpoint: "https://cloudcode-pa.googleapis.com/v1internal:onboardUser",
   loadCodeAssistUserAgent: "google-api-nodejs-client/9.15.1",
   loadCodeAssistApiClient: "google-cloud-sdk vscode_cloudshelleditor/0.1",
-  loadCodeAssistClientMetadata: `{"ideType":"IDE_UNSPECIFIED","platform":"PLATFORM_UNSPECIFIED","pluginType":"GEMINI"}`,
+  loadCodeAssistClientMetadata: JSON.stringify({ ideType: 9, platform: getOAuthPlatformEnum(), pluginType: 2 }),
 };
 
+/**
+ * Get client metadata using numeric enum values for API calls.
+ * @returns {{ ideType: number, platform: number, pluginType: number }}
+ */
+export function getOAuthClientMetadata() {
+  return { ideType: 9, platform: getOAuthPlatformEnum(), pluginType: 2 };
+}
+
 // OpenAI OAuth Configuration (Authorization Code Flow with PKCE)
 export const OPENAI_CONFIG = {
   clientId: "app_EMoamEEZ73f0CkXaXp7hrann",