fix: IPv6 URLs, cache-server SIGTERM and atomic task admission (#1217)

Three independent fixes, one commit each.

1. Hosts and ports were interpolated directly when building URLs, so a literal IPv6 address produced an unbracketed authority, making `ACTIONS_CACHE_URL`, `ACTIONS_RUNTIME_URL` and the artifact server listener unusable. Hosts are expected bare, as documented for `cache.host`, so an address that already carries brackets is no longer accepted.
2. `cache-server` waited on its own `os.Interrupt` channel and ignored SIGTERM, so service managers and container runtimes had to kill it.
3. Task admission used a separate `Load` and `Store`, so two concurrent dispatches of the same task id could both be admitted.

Assisted-by: Claude Code:Opus 5
Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1217
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

internal/app/cmd/cache-server.go

@@ -6,8 +6,6 @@ package cmd
 import (
 	"errors"
 	"fmt"
-	"os"
-	"os/signal"
 
 	"gitea.com/gitea/runner/act/artifactcache"
 	"gitea.com/gitea/runner/internal/app/run"
@@ -67,10 +65,8 @@ func runCacheServer(configFile *string, cacheArgs *cacheServerArgs) func(cmd *co
 
 		log.Infof("cache server is listening on %v", cacheHandler.ExternalURL())
 
-		c := make(chan os.Signal, 1)
-		signal.Notify(c, os.Interrupt)
-		<-c
+		<-cmd.Context().Done()
 
-		return nil
+		return cacheHandler.Close()
 	}
 }

internal/app/cmd/cmd.go

@@ -82,7 +82,7 @@ func Execute(ctx context.Context) {
 	// hide completion command
 	rootCmd.CompletionOptions.HiddenDefaultCmd = true
 
-	if err := rootCmd.Execute(); err != nil {
+	if err := rootCmd.ExecuteContext(ctx); err != nil {
 		os.Exit(1)
 	}
 }

internal/app/run/runner.go

@@ -281,10 +281,9 @@ func (r *Runner) SetCapabilitiesFromDeclare(resp *connect.Response[runnerv1.Decl
 }
 
 func (r *Runner) Run(ctx context.Context, task *runnerv1.Task) error {
-	if _, ok := r.runningTasks.Load(task.Id); ok {
+	if _, ok := r.runningTasks.LoadOrStore(task.Id, struct{}{}); ok {
 		return fmt.Errorf("task %d is already running", task.Id)
 	}
-	r.runningTasks.Store(task.Id, struct{}{})
 	defer r.runningTasks.Delete(task.Id)
 
 	r.runningCount.Add(1)