feat: add MusicBrainz artist-ID resolver

Add ResolveArtistMBID to query the MB artist search endpoint and return the
first matching artist ID, with httptest-backed tests for match, no-match,
HTTP error, and invalid-JSON paths.

internal/musicbrainz/resolve.go

@@ -0,0 +1,47 @@
+package musicbrainz
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/url"
+)
+
+// mbArtistSearchResult models the JSON response of the MusicBrainz artist
+// search endpoint (/ws/2/artist?query=artist:<name>&fmt=json). Only the
+// fields we need for MBID resolution are decoded.
+type mbArtistSearchResult struct {
+	Artists []struct {
+		ID   string `json:"id"`
+		Name string `json:"name"`
+		Score int   `json:"score"`
+	} `json:"artists"`
+}
+
+// ResolveArtistMBID resolves a MusicBrainz artist ID (MBID) for the given
+// artist name by querying the MusicBrainz artist search endpoint. It returns
+// the ID of the first (best-scoring) matching artist. An error is returned if
+// the search yields no matches, the response cannot be parsed, or the
+// underlying request fails.
+func (c *MusicBrainzClient) ResolveArtistMBID(ctx context.Context, name string) (string, error) {
+	params := url.Values{}
+	params.Set("query", fmt.Sprintf("artist:%s", name))
+	params.Set("fmt", "json")
+	path := "/artist?" + params.Encode()
+
+	body, err := c.doGet(ctx, path)
+	if err != nil {
+		return "", fmt.Errorf("resolve MBID for artist %q: %w", name, err)
+	}
+
+	var result mbArtistSearchResult
+	if err := json.Unmarshal(body, &result); err != nil {
+		return "", fmt.Errorf("parse artist search response for %q: %w", name, err)
+	}
+
+	if len(result.Artists) == 0 {
+		return "", fmt.Errorf("no MusicBrainz artist found for %q", name)
+	}
+
+	return result.Artists[0].ID, nil
+}

internal/musicbrainz/resolve_test.go

@@ -0,0 +1,105 @@
+package musicbrainz
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// newTestClient is defined in sync_test.go (signature: func newTestClient(serverURL string) *MusicBrainzClient).
+// contains is defined in model_test.go (signature: func contains(s []string, want string) bool).
+
+func TestResolveArtistMBID_MatchFound(t *testing.T) {
+	var gotPath string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotPath = r.URL.RequestURI()
+		w.Header().Set("Content-Type", "application/json")
+		_ = json.NewEncoder(w).Encode(mbArtistSearchResult{
+			Artists: []struct {
+				ID    string `json:"id"`
+				Name  string `json:"name"`
+				Score int    `json:"score"`
+			}{
+				{ID: "f27a7a7e-5a47-4cd5-afbe-6b7b01672b3b", Name: "Radiohead", Score: 100},
+				{ID: "another-id", Name: "Radiohead (Tribute)", Score: 80},
+			},
+		})
+	}))
+	defer server.Close()
+
+	client := newTestClient(server.URL)
+
+	mbid, err := client.ResolveArtistMBID(context.Background(), "Radiohead")
+	if err != nil {
+		t.Fatalf("ResolveArtistMBID() error = %v", err)
+	}
+
+	want := "f27a7a7e-5a47-4cd5-afbe-6b7b01672b3b"
+	if mbid != want {
+		t.Errorf("ResolveArtistMBID() = %q, want %q", mbid, want)
+	}
+
+	// Verify the request used the expected query path and JSON format.
+	if gotPath == "" || !strings.Contains(gotPath, "/artist?") {
+		t.Errorf("ResolveArtistMBID() requested path = %q, want /artist?...", gotPath)
+	}
+	if !strings.Contains(gotPath, "query=artist%3ARadiohead") {
+		t.Errorf("ResolveArtistMBID() query missing artist name, got %q", gotPath)
+	}
+	if !strings.Contains(gotPath, "fmt=json") {
+		t.Errorf("ResolveArtistMBID() query missing fmt=json, got %q", gotPath)
+	}
+
+}
+
+func TestResolveArtistMBID_NoMatch(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_ = json.NewEncoder(w).Encode(mbArtistSearchResult{Artists: []struct {
+			ID    string `json:"id"`
+			Name  string `json:"name"`
+			Score int    `json:"score"`
+		}{}})
+	}))
+	defer server.Close()
+
+	client := newTestClient(server.URL)
+
+	_, err := client.ResolveArtistMBID(context.Background(), "Nonexistent Artist 12345")
+	if err == nil {
+		t.Fatal("ResolveArtistMBID() expected error for no match, got nil")
+	}
+}
+
+func TestResolveArtistMBID_HTTPError(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusServiceUnavailable)
+		w.Write([]byte("Service Unavailable"))
+	}))
+	defer server.Close()
+
+	client := newTestClient(server.URL)
+
+	_, err := client.ResolveArtistMBID(context.Background(), "Radiohead")
+	if err == nil {
+		t.Fatal("ResolveArtistMBID() expected error on HTTP failure, got nil")
+	}
+}
+
+func TestResolveArtistMBID_InvalidJSON(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`not valid json`))
+	}))
+	defer server.Close()
+
+	client := newTestClient(server.URL)
+
+	_, err := client.ResolveArtistMBID(context.Background(), "Radiohead")
+	if err == nil {
+		t.Fatal("ResolveArtistMBID() expected parse error, got nil")
+	}
+}