test: cover response-less PapyrusError branch in health checks

[GeneralD]

カバレッジ調査の一環で見つけた、未カバーかつクリーンに埋められる分岐を補う test-only PR。

背景

MusicBrainzHealthCheck.healthCheck() / LRCLibHealthCheck.healthCheck() は、response を持たない PapyrusError（HTTP レスポンス到達前の接続レベル失敗）を HealthCheckResult(.fail, detail: error.message) にマッピングする。既存テストは .pass / HTTP コード / HTTP -1 / 汎用 Error を網羅していたが、response なしの PapyrusError だけ抜けており、この分岐（各ファイル46行目）が未カバーだった。

変更

• 両 suite に「response なし PapyrusError → error.message」ケースを追加
• region coverage: 各ファイル 75.00% → 87.50%（missed regions 4 → 2）

残る2 region は live な init() の URLSession クロージャ（実ネットワーク呼び出し）で、unit test では構造的にカバー不能。

調査メモ（カバレッジ全体）

ignore 対象（Domain/**・DependencyInjection/**）を除いた全体は line 96.4% / region 91.1% で既に高い。残るギャップの大半は OS 依存で unit test 困難:

領域	未カバー理由
Views/Overlay/AppWindow.swift	NSWindow / AVPlayerLayer の reconciliation（host-visual lane）
Views/Shared/DisplayLinkDriver.swift	CVDisplayLink
health check の live init()	実ネットワーク呼び出し
WallpaperDataSource/FindExecutable.swift の which fallback	Process spawn・PATH 依存（flaky）

→ この PR の health-check 分岐が、構造的でなく素直に埋められる数少ないケースだったため対応。

テスト

swift test 全 939 件 green（+2）。

バージョン

2.13.19 → 2.13.20（patch / test）

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📥 Commits

Reviewing files that changed from the base of the PR and between c455ac4 and b84d524.

📒 Files selected for processing (3)

• Sources/VersionHandler/Resources/version.txt
• Tests/LyricsDataSourceTests/LRCLibHealthCheckTests.swift
• Tests/MetadataDataSourceTests/MusicBrainzHealthCheckTests.swift

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• Commit unit tests in branch test/coverage-healthcheck-noresponse

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[codecov]

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