feat: implement stability fixes and enhance error handling

src/lib/antigravity/signature-utils.ts

@@ -0,0 +1,32 @@
+import { isEmpty } from 'lodash-es';
+/**
+ * Decode Base64-encoded signature if present
+ * Gemini sends Base64, Claude expects raw string
+ *
+ * @param signature - The signature string (may be Base64 or raw)
+ * @returns Decoded signature or undefined if input is empty
+ */
+export function decodeSignature(signature: string | undefined | null): string | undefined {
+  if (isEmpty(signature)) {
+    return undefined;
+  }
+
+  try {
+    const decoded = Buffer.from(signature!, 'base64').toString('utf-8');
+
+    // Validate: Check if it looks like valid UTF-8 text
+    // If decoding produces replacement characters, it wasn't valid Base64
+    if (decoded && !decoded.includes('\ufffd') && decoded.length > 0) {
+      // Additional check: re-encode and compare to detect false positives
+      const reEncoded = Buffer.from(decoded).toString('base64');
+      if (reEncoded === signature) {
+        return decoded;
+      }
+    }
+  } catch {
+    // Not valid Base64, return as-is
+  }
+
+  // Return original if not Base64
+  return signature!;
+}