test: proxy engine edge cases + storage/server minio retry

Cover denied, HEAD (cached/uncached/denied), stale-on-error, circuit-open,
and mutable revalidation (304) paths. proxy ~8->58%.

internal/server/server_test.go

@@ -11,6 +11,7 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+	"time"
 
 	"git.unkin.net/unkin/artifactapi/internal/config"
 	"git.unkin.net/unkin/artifactapi/internal/testsupport"
@@ -60,7 +61,13 @@ func TestMain(m *testing.M) {
 		S3Bucket:    "server-test",
 	}
 
-	srv, err := New(cfg, "test-version")
+	var srv *Server
+	for i := 0; i < 20; i++ { // tolerate MinIO reporting ready before bucket ops succeed
+		if srv, err = New(cfg, "test-version"); err == nil {
+			break
+		}
+		time.Sleep(500 * time.Millisecond)
+	}
 	if err != nil {
 		panic(err)
 	}