fix(core, web): keep App Check registered for secondary Firebase apps

[SelaseKay]

Description

On web, Firebase.initializeApp() activates App Check before other services so Auth can attach X-Firebase-AppCheck. The #11625 implementation did that with _services.remove('app-check'), which permanently dropped App Check from the registry after the first app.

For a secondary named app, Auth still ran ensurePluginInitialized (and accounts:lookup) without App Check being reactivated, so the App Check header was missing on reload when Auth enforcement was enabled.

This change keeps app-check registered, awaits its ensurePluginInitialized on every initializeApp, and still skips it in the parallel Future.wait so ordering is preserved.

Related Issues

• Fixes [firebase_app_check][web] X-Firebase-AppCheck missing on accounts:lookup after reload — still occurs with secondary (named) FirebaseApp + ReCaptchaEnterpriseProvider, post #11625 #18556

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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

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