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
+}