feat: add action_type and focal_node_id to AI chat message API

- Add VALID_ACTION_TYPES literal and action_type/focal_node_id fields to
  AIChatMessageRequest schema
- Add tree_id field to AIChatStartRequest schema for editor-embedded sessions
- Update send_message() signature with action_type and focal_node_id params
- Update start_chat_session() signature with tree_id param
- Pass new params through endpoints to service functions
- All new params have defaults so existing behavior is unchanged

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/schemas/ai_chat.py

@@ -14,12 +14,35 @@ class AIChatStartRequest(BaseModel):
     flow_type: Literal["troubleshooting", "procedural"] = Field(
         ..., description="Type of flow to build"
     )
+    tree_id: Optional[str] = Field(
+        default=None,
+        description="ID of existing tree for editor-embedded sessions",
+    )
+
+
+VALID_ACTION_TYPES = Literal[
+    "generate_full",
+    "generate_branch",
+    "modify_node",
+    "add_steps",
+    "quick_action",
+    "open_chat",
+    "variable_inference",
+]
 
 
 class AIChatMessageRequest(BaseModel):
     """Send a user message in a chat session."""
 
     content: str = Field(..., min_length=1, max_length=5000)
+    action_type: Optional[VALID_ACTION_TYPES] = Field(
+        default="open_chat",
+        description="Type of AI action to perform",
+    )
+    focal_node_id: Optional[str] = Field(
+        default=None,
+        description="ID of the node/step being acted on",
+    )
 
 
 class AIChatImportRequest(BaseModel):