fix: skip LLM-based title generation for ACP agents

The ACPAgent uses a sentinel LLM with model="acp-managed" that cannot
make direct LLM calls. When generate_title() was called for ACP
conversations, it would fall back to this sentinel LLM and fail with:
"LiteLLM Provider NOT provided. You passed model=acp-managed"

Add a guard to detect the sentinel model and pass None to
generate_conversation_title(), which causes it to use simple truncation
fallback instead of LLM-based title generation.

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

Author: simonrosenberg

openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py

@@ -911,6 +911,11 @@ def generate_title(self, llm: LLM | None = None, max_length: int = 50) -> str:
         # Use provided LLM or fall back to agent's LLM
         llm_to_use = llm or self.agent.llm
 
+        # Skip LLM-based title generation for ACP agents with sentinel LLM
+        # The sentinel model "acp-managed" cannot make LLM calls directly
+        if llm_to_use.model == "acp-managed":
+            llm_to_use = None
+
         return generate_conversation_title(
             events=self._state.events, llm=llm_to_use, max_length=max_length
         )