🤖 fix: VS Code live workspace list follow-ups (refresh race, reconnect, multi-project filter) #5654

Follow-up from the final readiness check of #5651 (#5109 PR2a, live workspace list in the VS Code host). None of these blocks #5651; each needs its own small change.

1. Refresh race on a connection change: if the server URL or auth token changes while a refresh waits on getWorkspacesForSidebar, that refresh can still finish and open a subscription on the old client. Fix: bump refreshWorkspacesGeneration where the setting and secret listeners call dropValidatedApi().
2. No reconnect: if the metadata stream errors (server restart, network loss), the list stays static until the next manual refresh, and the error is logged only at debug level. Fix: restart the subscription with backoff, or trigger a refresh.
3. Hidden multi-project workspaces: with the multi-project experiment off, workspace.list and the snapshot hide _multi workspaces, but per-workspace update events do not appear to apply that filter (not verified end to end). Check and filter updates the same way.
4. Optional tests: an update that removes the selected workspace clears the selection; file mode opens no subscription.

Owner: issue coordinator lane (workspace-74). Trigger: after the #5109 stack merges.

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