fix(resilience): complete circuit breaker patterns per PR review

Address bot review feedback on PR #32:

- Add _disk_available flag update in _write_json exception handler

- Add _disk_available flag update in log_stream_chunk (critical for streams)

- Document intentional no-memory-fallback design for streams

- Add _fallback_mode update in failure_logger exception handler

- Add complete circuit breaker pattern to usage_manager

src/proxy_app/detailed_logger.py

@@ -50,6 +50,7 @@ class DetailedLogger:
             with open(self.log_dir / filename, "w", encoding="utf-8") as f:
                 json.dump(data, f, indent=4, ensure_ascii=False)
         except (OSError, PermissionError, IOError) as e:
+            DetailedLogger._disk_available = False
             logging.error(f"[{self.request_id}] Failed to write to {filename}: {e}")
             self._in_memory_logs.append({"file": filename, "data": data})
 
@@ -66,8 +67,9 @@ class DetailedLogger:
 
     def log_stream_chunk(self, chunk: Dict[str, Any]):
         """Logs an individual chunk from a streaming response to a JSON Lines file."""
+        # Intentionally skip memory fallback for streams to prevent OOM - unlike _write_json, we don't buffer stream chunks in memory
         if not DetailedLogger._disk_available:
-            return  # Skip chunk logging when disk unavailable
+            return
         
         try:
             self.log_dir.mkdir(parents=True, exist_ok=True)
@@ -78,6 +80,7 @@ class DetailedLogger:
             with open(self.log_dir / "streaming_chunks.jsonl", "a", encoding="utf-8") as f:
                 f.write(json.dumps(log_entry, ensure_ascii=False) + "\n")
         except (OSError, PermissionError, IOError) as e:
+            DetailedLogger._disk_available = False
             logging.error(f"[{self.request_id}] Failed to write stream chunk: {e}")
 
     def log_final_response(self, status_code: int, headers: Optional[Dict[str, Any]], body: Dict[str, Any]):

src/rotator_library/failure_logger.py

@@ -194,6 +194,8 @@ def log_failure(
     try:
         failure_logger.error(detailed_log_data)
     except (OSError, IOError) as e:
+        global _fallback_mode
+        _fallback_mode = True
         # File logging failed - log to console instead
         logging.error(f"Failed to write to failures.log: {e}")
         logging.error(f"Failure summary: {summary_message}")

src/rotator_library/usage_manager.py

@@ -72,6 +72,9 @@ class UsageManager:
 
         self._timeout_lock = asyncio.Lock()
         self._claimed_on_timeout: Set[str] = set()
+        
+        # Circuit breaker for disk write failures
+        self._disk_available = True
 
         if daily_reset_time_utc:
             hour, minute = map(int, daily_reset_time_utc.split(":"))
@@ -113,6 +116,9 @@ class UsageManager:
             except (OSError, PermissionError, IOError) as e:
                 lib_logger.warning(f"Cannot read usage file {self.file_path}: {e}. Using empty state.")
                 self._usage_data = {}
+            else:
+                # [CIRCUIT BREAKER RESET] Successfully loaded, re-enable disk writes
+                self._disk_available = True
 
     async def _save_usage(self):
         """Saves the current usage data to the JSON file asynchronously with resilience.
@@ -123,6 +129,9 @@ class UsageManager:
         """
         if self._usage_data is None:
             return
+        
+        if not self._disk_available:
+            return  # Skip disk write when unavailable
 
         try:
             async with self._data_lock:
@@ -134,6 +143,8 @@ class UsageManager:
                 async with aiofiles.open(self.file_path, "w") as f:
                     await f.write(json.dumps(self._usage_data, indent=2))
         except (OSError, PermissionError, IOError) as e:
+            # [CIRCUIT BREAKER] Disable disk writes to prevent repeated failures
+            self._disk_available = False
             # [FAIL SILENTLY, LOG LOUDLY] Log the error but don't crash
             # In-memory state is preserved and will continue to work
             lib_logger.warning(