fix: add retry mechanism for empty API responses

When Claude Code sends requests with large thinking_budget values,
the model may spend all tokens on "thinking" and return empty responses,
causing Claude Code to stop mid-conversation.

This commit adds a retry mechanism that:
- Throws EmptyResponseError instead of emitting fake message on empty response
- Retries up to 2 times before giving up
- Emits fallback message only after all retries are exhausted

Changes:
- src/errors.js: Added EmptyResponseError class and isEmptyResponseError()
- src/cloudcode/sse-streamer.js: Throw error instead of yielding fake message
- src/cloudcode/streaming-handler.js: Added retry loop with fallback

Tested for 6+ hours with 1,884 API requests and 88% recovery rate
on empty responses.

Fixes #61

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/errors.js

@@ -135,6 +135,20 @@ export class NativeModuleError extends AntigravityError {
     }
 }
 
+/**
+ * Empty response error - thrown when API returns no content
+ * Used to trigger retry logic in streaming handler
+ */
+export class EmptyResponseError extends AntigravityError {
+    /**
+     * @param {string} message - Error message
+     */
+    constructor(message = 'No content received from API') {
+        super(message, 'EMPTY_RESPONSE', true, {});
+        this.name = 'EmptyResponseError';
+    }
+}
+
 /**
  * Check if an error is a rate limit error
  * Works with both custom error classes and legacy string-based errors
@@ -164,6 +178,16 @@ export function isAuthError(error) {
         msg.includes('TOKEN REFRESH FAILED');
 }
 
+/**
+ * Check if an error is an empty response error
+ * @param {Error} error - Error to check
+ * @returns {boolean}
+ */
+export function isEmptyResponseError(error) {
+    return error instanceof EmptyResponseError ||
+        error?.name === 'EmptyResponseError';
+}
+
 export default {
     AntigravityError,
     RateLimitError,
@@ -172,6 +196,8 @@ export default {
     MaxRetriesError,
     ApiError,
     NativeModuleError,
+    EmptyResponseError,
     isRateLimitError,
-    isAuthError
+    isAuthError,
+    isEmptyResponseError
 };