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/provider/npm/npm_test.go

@@ -0,0 +1,78 @@
+package npm
+
+import (
+	"context"
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func TestType(t *testing.T) {
+	if (&Provider{}).Type() != models.PackageNPM {
+		t.Fatal("wrong type")
+	}
+}
+
+func TestClassify(t *testing.T) {
+	p := &Provider{}
+	if p.Classify("pkg/-/pkg-1.0.0.tgz") != provider.Immutable {
+		t.Error("tgz should be immutable")
+	}
+	if p.Classify("pkg") != provider.Mutable {
+		t.Error("metadata should be mutable")
+	}
+}
+
+func TestContentType(t *testing.T) {
+	p := &Provider{}
+	if p.ContentType("pkg/-/pkg-1.0.0.tgz") != "application/gzip" {
+		t.Error("tgz content type")
+	}
+	if p.ContentType("pkg") != "application/json" {
+		t.Error("metadata content type")
+	}
+}
+
+func TestUpstreamURL(t *testing.T) {
+	p := &Provider{}
+	got := p.UpstreamURL(models.Remote{BaseURL: "https://registry.npmjs.org/"}, "/pkg")
+	if got != "https://registry.npmjs.org/pkg" {
+		t.Errorf("got %q", got)
+	}
+}
+
+func TestRewriteResponse(t *testing.T) {
+	p := &Provider{}
+	remote := models.Remote{Name: "npmjs", BaseURL: "https://registry.npmjs.org"}
+
+	if out, _ := p.RewriteResponse([]byte(`{"a":1}`), remote, ""); out != nil {
+		t.Error("empty proxyBaseURL should be a no-op")
+	}
+	if out, _ := p.RewriteResponse([]byte("not json"), remote, "http://proxy"); out != nil {
+		t.Error("invalid json should be a no-op")
+	}
+	body := []byte(`{"tarball":"https://registry.npmjs.org/pkg/-/pkg-1.0.0.tgz"}`)
+	out, err := p.RewriteResponse(body, remote, "http://proxy")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(out) != `{"tarball":"http://proxy/api/v1/remote/npmjs/pkg/-/pkg-1.0.0.tgz"}` {
+		t.Errorf("rewrite: %s", out)
+	}
+	if out, _ := p.RewriteResponse([]byte(`{"x":"unrelated"}`), remote, "http://proxy"); out != nil {
+		t.Error("no matching base URL should be a no-op")
+	}
+}
+
+func TestAuthHeaders(t *testing.T) {
+	p := &Provider{}
+	h, _ := p.AuthHeaders(context.Background(), models.Remote{Username: "u", Password: "pw"})
+	if h.Get("Authorization") == "" {
+		t.Error("expected auth header when credentials set")
+	}
+	h, _ = p.AuthHeaders(context.Background(), models.Remote{})
+	if h.Get("Authorization") != "" {
+		t.Error("expected no auth header without credentials")
+	}
+}