Add unit test for FirebaseOptions copying

[google-labs-jules]

Added testCopying to FirebaseCore/Tests/SwiftUnit/FirebaseOptionsTests.swift. This test ensures that FirebaseOptions instances are copied correctly, covering both dictionary-backed properties and the appGroupID property which requires manual copying logic. This prevents regressions where new properties might be missed in the copyWithZone implementation.

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PR created automatically by Jules for task 12823941451683876584 started by @ryanwilson

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[danger-firebase-ios]

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