admin: fix manual job run to use scheduler dispatch with capacity management and retry (#8720)

RunPluginJobTypeAPI previously executed proposals with a naive sequential loop
calling ExecutePluginJob per proposal. This had two bugs:

1. Double-lock: RunPluginJobTypeAPI held pluginLock while calling ExecutePluginJob,
   which tried to re-acquire the same lock for every job in the loop.

2. No capacity management: proposals were fired directly at workers without
   reserveScheduledExecutor, so every job beyond the worker concurrency limit
   received an immediate at_capacity error with no retry or backoff.

Fix: add Plugin.DispatchProposals which reuses dispatchScheduledProposals - the
same code path the scheduler loop uses - with executor reservation, configurable
concurrency, and per-job retry with backoff. RunPluginJobTypeAPI now calls
DispatchPluginProposals (a thin AdminServer wrapper) after holding pluginLock once.

Co-authored-by: Anton Ustyugov <anton@devops>

weed/admin/dash/admin_server.go

@@ -1284,6 +1284,23 @@ func (s *AdminServer) RunPluginDetectionWithReport(
 	return s.plugin.RunDetectionWithReport(ctx, jobType, clusterContext, maxResults)
 }
 
+// DispatchPluginProposals dispatches a batch of proposals using the same
+// capacity-aware dispatch logic as the scheduler loop (executor reservation with
+// backoff, per-job retry on transient errors). The plugin lock must already be
+// held by the caller.
+func (s *AdminServer) DispatchPluginProposals(
+	ctx context.Context,
+	jobType string,
+	proposals []*plugin_pb.JobProposal,
+	clusterContext *plugin_pb.ClusterContext,
+) (successCount, errorCount, canceledCount int, err error) {
+	if s.plugin == nil {
+		return 0, 0, 0, fmt.Errorf("plugin is not enabled")
+	}
+	sc, ec, cc := s.plugin.DispatchProposals(ctx, jobType, proposals, clusterContext)
+	return sc, ec, cc, nil
+}
+
 // ExecutePluginJob dispatches one job to a capable worker and waits for completion.
 func (s *AdminServer) ExecutePluginJob(
 	ctx context.Context,