fix: add conversation_id support to LettaAgentV3 constructor (#9156)

**Error:**
```
TypeError: LettaAgentV2.__init__() got an unexpected keyword argument 'conversation_id'
```

**Trace:** https://letta.grafana.net/goto/afbk4da3fuxhcf?orgId=stacks-1189126

**Problem:**
The `POST /v1/conversations/{conversation_id}/compact` endpoint was failing
because `LettaAgentV3` inherits from `LettaAgentV2` without overriding
`__init__`, so passing `conversation_id` to the constructor failed.

**Fix:**
1. Add `__init__` to `LettaAgentV3` that accepts optional `conversation_id`
2. Remove redundant `conversation_id` param from `_checkpoint_messages` -
   use `self.conversation_id` consistently instead
3. Clean up internal callers that were passing `conversation_id=self.conversation_id`

Backward compatible - existing code creating `LettaAgentV3(agent_state, actor)`
still works since `conversation_id` defaults to `None`.

👾 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

letta/agents/letta_agent_v3.py

@@ -41,6 +41,7 @@ from letta.schemas.step import StepProgression
 from letta.schemas.step_metrics import StepMetrics
 from letta.schemas.tool_execution_result import ToolExecutionResult
 from letta.schemas.usage import LettaUsageStatistics
+from letta.schemas.user import User
 from letta.server.rest_api.utils import (
     create_approval_request_message_from_llm_response,
     create_letta_messages_from_llm_response,
@@ -72,6 +73,16 @@ class LettaAgentV3(LettaAgentV2):
     * Support Gemini / OpenAI client
     """
 
+    def __init__(
+        self,
+        agent_state: AgentState,
+        actor: User,
+        conversation_id: str | None = None,
+    ):
+        super().__init__(agent_state, actor)
+        # Set conversation_id after parent init (which calls _initialize_state)
+        self.conversation_id = conversation_id
+
     def _initialize_state(self):
         super()._initialize_state()
         self._require_tool_call = False
@@ -485,9 +496,7 @@ class LettaAgentV3(LettaAgentV2):
                 context_window=self.agent_state.llm_config.context_window,
             )
 
-    async def _checkpoint_messages(
-        self, run_id: str, step_id: str, new_messages: list[Message], in_context_messages: list[Message], conversation_id: str | None = None
-    ):
+    async def _checkpoint_messages(self, run_id: str, step_id: str, new_messages: list[Message], in_context_messages: list[Message]):
         """
         Checkpoint the current message state - run this only when the current messages are 'safe' - meaning the step has completed successfully.
 
@@ -506,7 +515,7 @@ class LettaAgentV3(LettaAgentV2):
         for message in new_messages:
             message.step_id = step_id
             message.run_id = run_id
-            message.conversation_id = conversation_id
+            message.conversation_id = self.conversation_id
 
         # persist the new message objects - ONLY place where messages are persisted
         persisted_messages = await self.message_manager.create_many_messages_async(
@@ -808,7 +817,6 @@ class LettaAgentV3(LettaAgentV2):
                                 step_id=step_id,
                                 new_messages=[summary_message],
                                 in_context_messages=messages,
-                                conversation_id=self.conversation_id,
                             )
 
                         else:
@@ -878,7 +886,6 @@ class LettaAgentV3(LettaAgentV2):
                 step_id=step_id,
                 new_messages=input_messages_to_persist + new_messages,
                 in_context_messages=messages,  # update the in-context messages
-                conversation_id=self.conversation_id,
             )
 
             # yield back generated messages
@@ -928,7 +935,6 @@ class LettaAgentV3(LettaAgentV2):
                     step_id=step_id,
                     new_messages=[summary_message],
                     in_context_messages=messages,
-                    conversation_id=self.conversation_id,
                 )
 
         except Exception as e: