fix(core): reset length_continue_retries after successful continuation

length_continue_retries and truncated_response_prefix were initialized
once before the outer loop and never reset after a successful
continuation. If a conversation hit length truncation once (counter=1),
succeeded on continuation, did more tool calls, then hit length again,
the counter started at 1 instead of 0 — reducing available retries
from 3 to 2. The stale truncated_response_prefix would also leak
into the next response.

Reset both after the prefix is consumed on a successful final response.

run_agent.py

@@ -6169,6 +6169,8 @@ class AIAgent:
 
                     if truncated_response_prefix:
                         final_response = truncated_response_prefix + final_response
+                        truncated_response_prefix = ""
+                        length_continue_retries = 0
                     
                     # Strip <think> blocks from user-facing response (keep raw in messages for trajectory)
                     final_response = self._strip_think_blocks(final_response).strip()