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>

open-sse/utils/sessionManager.js

@@ -0,0 +1,66 @@
+/**
+ * Session Manager for Antigravity Cloud Code
+ *
+ * Handles session ID generation and caching for prompt caching continuity.
+ * Mimics the Antigravity binary behavior: generates a session ID at startup
+ * and keeps it for the process lifetime, scoped per account/connection.
+ *
+ * Reference: antigravity-claude-proxy/src/cloudcode/session-manager.js
+ */
+
+import crypto from "crypto";
+
+// Runtime storage for session IDs (per connection/account)
+// Key: connectionId (email or identifier), Value: sessionId
+const runtimeSessionStore = new Map();
+
+/**
+ * Get or create a session ID for the given connection.
+ *
+ * The binary generates a session ID once at startup: `rs() + Date.now()`.
+ * Since 9router is long-running, we simulate this "per-launch" behavior by
+ * storing a generated ID in memory for each connection.
+ *
+ * - If 9router restarts, the ID changes (matching binary restart behavior).
+ * - Within a running instance, the ID is stable for that connection.
+ * - This enables prompt caching while using the EXACT random logic of the binary.
+ *
+ * @param {string} connectionId - The connection identifier (email or unique ID)
+ * @returns {string} A stable session ID string matching binary format
+ */
+export function deriveSessionId(connectionId) {
+    if (!connectionId) {
+        // Fallback for requests without a connection identifier
+        return generateBinaryStyleId();
+    }
+
+    // Check if we already have a session ID for this connection in this process run
+    if (runtimeSessionStore.has(connectionId)) {
+        return runtimeSessionStore.get(connectionId);
+    }
+
+    // Generate a new ID using the binary's exact logic
+    const newSessionId = generateBinaryStyleId();
+
+    // Store it for future requests from this connection
+    runtimeSessionStore.set(connectionId, newSessionId);
+
+    return newSessionId;
+}
+
+/**
+ * Generate a Session ID using the binary's exact logic.
+ * Format: `rs() + Date.now()` where `rs()` is randomUUID
+ *
+ * @returns {string} A session ID in binary format
+ */
+export function generateBinaryStyleId() {
+    return crypto.randomUUID() + Date.now().toString();
+}
+
+/**
+ * Clears all session IDs (e.g. useful for testing or explicit reset)
+ */
+export function clearSessionStore() {
+    runtimeSessionStore.clear();
+}