feat(agent): improve tiktoken initialization and device registration

- Configure tiktoken cache directory to isolate encoding files
- Add logging and timing for tiktoken initialization process
- Make Huawei CE and Ruijie Telnet device registration idempotent
- Add duplicate registration prevention with global flags
- Add logging for device type registration status
- Update .gitignore to exclude tiktoken cache files

.gitignore

@@ -72,3 +72,6 @@ venv
 .claude/
 !.claude/development.md  # Exception: allow development docs
 # PROJECT_CONTEXT.md  # Commented out: allow tracking project context
+
+# Tiktoken cache files
+gns3server/agent/gns3_copilot/cache/tiktoken/