refactor: replace _previous_leased with derived LeaseState

Introduce LeaseState enum and _lease_state property derived from
_lease_context, eliminating a class of state-synchronization bugs
where _previous_leased could drift from _lease_context.

Key changes:
- Wrap handle_lease body in try/finally so early returns (stale lease,
  session setup race) always clean up _lease_context and log context
- Reject overlapping leases in _apply_status instead of silently
  replacing _lease_context, preventing concurrent handle_lease tasks
- Move before-lease hook spawn into _on_lease_acquired for cleaner
  ownership of lease startup
- Remove stale _previous_leased from test fixtures

Signed-off-by: Benny Zlotnik <bzlotnik@redhat.com>
Assited-by: claude-opus-4.6

Author: bennyz