feat: add Pydantic schemas for branching, handoffs, and resolution outputs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/ai_session.py

@@ -228,6 +228,8 @@ class AISessionDetail(AISessionSummary):
     ticket_data: dict[str, Any] | None = None
     steps: list[AISessionStepResponse] = []
     conversation_messages: list[dict[str, Any]] = []  # Chat sessions store messages here
+    is_branching: bool = False
+    active_branch_id: str | None = None
 
     model_config = {"from_attributes": True}