Strip codicon references from session descriptions displayed as plaintext

Fixes codicon syntax like $(lock) and other markdown artifacts showing
as raw text in the sessions list, quick picker, and title bar.

Uses stripIcons() from iconLabels.ts following the existing pattern
in chatToolProgressPart.ts, chatProgressContentPart.ts, and
chatResponseAccessibleView.ts.

Agent-Logs-Url: https://github.com/microsoft/vscode/sessions/754734aa-4ba7-4869-b9e5-b358b2e826cd

Co-authored-by: bpasero <[email protected]>