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/cloudcode/sse-streamer.js

@@ -7,6 +7,7 @@
 
 import crypto from 'crypto';
 import { MIN_SIGNATURE_LENGTH, getModelFamily } from '../constants.js';
+import { EmptyResponseError } from '../errors.js';
 import { cacheSignature, cacheThinkingSignature } from '../format/signature-cache.js';
 import { logger } from '../utils/logger.js';
 
@@ -226,39 +227,10 @@ export async function* streamSSEResponse(response, originalModel) {
         }
     }
 
-    // Handle no content received
+    // Handle no content received - throw error to trigger retry in streaming-handler
     if (!hasEmittedStart) {
-        logger.warn('[CloudCode] No content parts received, emitting empty message');
-        yield {
-            type: 'message_start',
-            message: {
-                id: messageId,
-                type: 'message',
-                role: 'assistant',
-                content: [],
-                model: originalModel,
-                stop_reason: null,
-                stop_sequence: null,
-                usage: {
-                    input_tokens: inputTokens - cacheReadTokens,
-                    output_tokens: 0,
-                    cache_read_input_tokens: cacheReadTokens,
-                    cache_creation_input_tokens: 0
-                }
-            }
-        };
-
-        yield {
-            type: 'content_block_start',
-            index: 0,
-            content_block: { type: 'text', text: '' }
-        };
-        yield {
-            type: 'content_block_delta',
-            index: 0,
-            delta: { type: 'text_delta', text: '[No response received from API]' }
-        };
-        yield { type: 'content_block_stop', index: 0 };
+        logger.warn('[CloudCode] No content parts received, throwing for retry');
+        throw new EmptyResponseError('No content parts received from API');
     } else {
         // Close any open block
         if (currentBlockType !== null) {