fix(core,web): preserve non-JavaScript app errors

[SelaseKay]

Description

FirebaseCoreWeb.app() and secondary-app initialization cast every caught exception to JSError, which could mask Dart exceptions produced by guardNotInitialized with an unrelated TypeError. This change rethrows non-JavaScript exceptions unchanged while preserving the existing Firebase JavaScript error mapping, and adds a browser regression test for [core/not-initialized].

Verified with flutter test --platform chrome and flutter analyze in packages/firebase_core/firebase_core_web.

Related Issues

• Fixes [firebase_core_web]: FirebaseCoreWeb.app() masks the real error with an unsafe as JSError cast (same pattern as #18176, not covered by that fix) #18548

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