fix: initialize parsed_response to prevent UnboundLocalError crashing all streaming responses

parsed_response was only assigned inside the memory_enabled block but
referenced unconditionally by CCR feedback and prefix cache tracker in
the finally block, causing every non-memory streaming request to crash
with UnboundLocalError and abort the connection.

headroom/proxy/handlers/streaming.py

@@ -523,6 +523,7 @@ class StreamingMixin:
             # For memory mode, we buffer the response to check for tool calls
             buffered_chunks: list[bytes] = []
             full_sse_data = ""
+            parsed_response = None  # Set by memory block; used by CCR + prefix tracker
 
             try:
                 async with contextlib.aclosing(upstream_response) as response: