fix(pier): reap stateful sessions whose store record expired

- a reaper started at Mount checks every local session against the store
  every min(idle_ttl/2, 1m) and closes each copy whose record is gone,
  logging the expired step (A-CR-01, A-IN-06)
- the GET stream re-check ends the stream when the record is gone (A-IN-01)
- tool calls served without a session on the stateful path log the
  session as ephemeral (A-IN-06)
- TestPierReaperSmoke and the ReapNow, LocalSessions, StopReaper seams

modules/pier/export_test.go

@@ -49,3 +49,21 @@ func (p *Pier) StatelessServers() int {
 
 // SetRecheck sets how often a long GET stream re-verifies its bearer.
 func (p *Pier) SetRecheck(d time.Duration) { p.recheck = d }
+
+// ReapNow runs one session reaper pass and answers how many local sessions
+// it closed.
+func (p *Pier) ReapNow(ctx context.Context) int { return p.sessions.reapOnce(ctx) }
+
+// LocalSessions counts the stateful sessions connected on this instance.
+func (p *Pier) LocalSessions() int {
+	p.sessions.mu.Lock()
+	defer p.sessions.mu.Unlock()
+	return len(p.sessions.local)
+}
+
+// StopReaper ends the session reaper goroutine Mount started.
+func (p *Pier) StopReaper() {
+	if p.stopReap != nil {
+		p.stopReap()
+	}
+}