A session with messages belongs to the profile it was created under. Switching
profiles while a conversation is in progress should not retag that session or
update its workspace/model in place — that would corrupt the session's context.
New behavior:
- Session has NO messages (empty): profile switch updates it in place (model,
workspace). Works exactly as before — nothing was started yet.
- Session HAS messages (in progress): profile switch calls newSession() to
start a fresh session tagged to the new profile. The old session is left
untouched. Toast: 'Switched to profile: X — new conversation started'.
- Agent busy: blocked as before, no change.
Also: S._profileDefaultWorkspace is now consumed (set to null) inside
newSession() after the first use, so it doesn't keep forcing the same
workspace on every subsequent new session after a switch.
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