admin: make the cluster lock's fairness window configurable (#12426)

* admin: make the cluster lock's fairness window configurable

Scheduled jobs hold the cluster admin lock for their whole run. Once the
lock had been held for a minute, every new job waited until all running
jobs had released it, so with hour-long EC jobs dispatch ran in waves and
most workers sat idle between them.

-adminLock.fairnessWindow keeps the one-minute default; 0 lets new jobs
join the shared hold, and shell clients get the lock when the scheduler
next drains.

Claude-Session: https://claude.ai/code/session_011UpvafcfUuC2mLxNAXLQJi

* admin: wait for the blocked acquire before disabling the fairness window

The wake-up test slept 50 ms before SetFairnessWindow(0), so a late
acquire that started after the change joined without exercising the
Broadcast. Wait until it is registered as a waiter instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

weed/admin/dash/admin_server.go

@@ -1708,6 +1708,15 @@ func (s *AdminServer) GetPlugin() *adminplugin.Plugin {
 	return s.plugin
 }
 
+// SetAdminLockFairnessWindow sets the cluster admin lock's fairness window
+// (see AdminLockManager.SetFairnessWindow).
+func (s *AdminServer) SetAdminLockFairnessWindow(window time.Duration) {
+	if s == nil || s.pluginLock == nil {
+		return
+	}
+	s.pluginLock.SetFairnessWindow(window)
+}
+
 func (s *AdminServer) acquirePluginLock(reason string) (func(), error) {
 	if s == nil || s.pluginLock == nil {
 		return func() {}, nil