Feature: Ephemeral action runners (#33570)

* This includes a runner mock test for hardend PickTask behavior like
described in my proposal
* Runner register ephemeral flag introduced in
https://gitea.com/gitea/act_runner/pulls/649

Closes #32461

models/migrations/v1_24/v315.go

@@ -0,0 +1,16 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_24 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddEphemeralToActionRunner(x *xorm.Engine) error {
+	type ActionRunner struct {
+		Ephemeral bool `xorm:"ephemeral NOT NULL DEFAULT false"`
+	}
+
+	return x.Sync(new(ActionRunner))
+}