fix: release broker state during dead session cleanup

[ejgallego]

This PR prevents cleanup of an exited backend worker from blocking unrelated Beam workspaces while preserving workspace-generation isolation.

• Atomically detach an exited session before performing its bounded shutdown outside the broker state mutex.
• Retry only while the request's original workspace generation remains current; reject a concurrent reset with contentModified instead of touching the replacement workspace.
• Cover both inherited stdio held by a worker descendant and reset during the resulting cleanup window.