fix: return pending_task_lane in session detail API response + debug logging

_build_session_detail was omitting pending_task_lane, is_branching, and
active_branch_id from the GET /ai-sessions/{id} response. The fields
existed on the schema and model but were never passed in the manual
constructor, so task lane state could never be restored on navigation.

Also adds console logging to AssistantChatPage selectChat flow to
diagnose message restoration, and fixes ScriptTemplateEditor stepper
dismiss firing during programmatic script_body updates.

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

backend/app/api/endpoints/ai_sessions.py

@@ -117,6 +117,9 @@ def _build_session_detail(session: AISession) -> AISessionDetail:
         resolved_at=session.resolved_at,
         steps=step_responses,
         conversation_messages=session.conversation_messages or [],
+        pending_task_lane=session.pending_task_lane,
+        is_branching=getattr(session, 'is_branching', False),
+        active_branch_id=str(session.active_branch_id) if getattr(session, 'active_branch_id', None) else None,
     )