fix: 正常終了時のWebSocketエラーログを抑制 (suppress shutdown errors)

[haranishi]

正常な browser.close() の終了処理で発生するWebSocketのerrorイベントが、console.errorとして記録される問題を修正します。Browser.closeの送信、transportのclose、WebSocketのcloseイベントで終了状態を記録し、終了処理中のerrorログを抑制します。稼働中のエラーは引き続き記録します。

認証情報や実ブラウザーを必要としない5つの回帰テストと、テスト用CIを追加しました。

Normal browser.close() can emit WebSocket error events during shutdown and create misleading error logs. Track shutdown when sending Browser.close, closing the transport, or receiving a close event, and suppress error logs once shutdown starts. Errors in active sessions remain visible.

Add five credential-free regression tests and a dedicated pull-request workflow.

Validation:

• Before the fix: three shutdown cases failed; the two active-error cases passed.
• After the fix: all five unit tests passed (npm run test:unit in packages/playwright-cloudflare/tests).
• npm run build in packages/playwright-cloudflare passed.
• ESLint and a TypeScript check of the changed source/test files passed.
• Remote Browser Run end-to-end tests were not run; regression coverage uses a mock WebSocket.

Closes #230.

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