test: unit tests for pure provider/config/auth packages

First increment toward 90% core-package coverage. Adds Docker-free unit
tests taking these packages to full or near-full coverage:
- provider/npm, provider/alpine, provider/puppet: 100%
- provider/pypi: index generation via a fake FileStore, upload validation,
  name parsing, classification, rewrite
- config: defaults, overrides, DSN, invalid port
- auth: basic header with/without credentials

Infra-backed packages (database, storage, cache, proxy engine, api
handlers, server) still need the testcontainers batch to reach 90%.

internal/auth/basic_test.go

@@ -0,0 +1,23 @@
+package auth
+
+import (
+	"encoding/base64"
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func TestBasicHeaders(t *testing.T) {
+	h := BasicHeaders(models.Remote{Username: "alice", Password: "secret"})
+	got := h.Get("Authorization")
+	want := "Basic " + base64.StdEncoding.EncodeToString([]byte("alice:secret"))
+	if got != want {
+		t.Errorf("Authorization = %q, want %q", got, want)
+	}
+}
+
+func TestBasicHeadersNoUser(t *testing.T) {
+	if h := BasicHeaders(models.Remote{}); h.Get("Authorization") != "" {
+		t.Error("expected no Authorization header without a username")
+	}
+}