fix(vscode): prevent UI blocking when switching sessions rapidly (#533)

* fix(vscode): prevent UI blocking when switching sessions rapidly

- Show loading spinner in MessageList during session switch instead of blank screen
- Fire handleLoadMessages without await so rapid switches aren't serialized
- Abort stale getMessages requests via AbortController when switching away
- Scope error messages with sessionID so stale 404s don't clear wrong session's loading state
- Silence expected 404s on cross-worktree getSession calls via silent HTTP option

* fix: replace empty catch block with warning log for getSession errors