Phase 4-6: Dead code removal, performance, minor fixes

Phase 4:
- Remove legacy SessionRecord, _sessions, _msg_to_session from SessionStore
- Fix hardcoded provider in root endpoint (use settings.provider_type)
- Update session store tests

Phase 5:
- Use list-based string accumulation in ThinkingSegment, TextSegment, ToolCallSegment
- Cache MAX_MESSAGE_LOG_ENTRIES_PER_CHAT at SessionStore init
- Use iterative DFS in MessageTree.get_descendants

Phase 6:
- Add comment for abstract async generator workaround in BaseProvider
- Rename TELEGRAM_EDIT log tags to PLATFORM_EDIT in handler

Co-authored-by: Ali Khokhar <alishahryar2@gmail.com>

api/routes.py

@@ -100,7 +100,7 @@ async def root(settings: Settings = Depends(get_settings)):
     """Root endpoint."""
     return {
         "status": "ok",
-        "provider": "nvidia_nim",
+        "provider": settings.provider_type,
         "model": settings.model,
     }