Restore CI warnings-as-errors for FirebaseAuth pod tests with Xcode 16

[paulb777]

Description

- ERROR | xcodebuild:  /Users/runner/work/firebase-ios-sdk/firebase-ios-sdk/FirebaseAuth/Sources/Swift/Utilities/AuthWebViewController.swift:19:3: error: add '@preconcurrency' to suppress 'Sendable'-related warnings from module 'WebKit'

Reproducing the issue

See workaround in FirebaseAuth.podspec and https://github.com/firebase/firebase-ios-sdk/actions/runs/11003726045/job/30553328769#step:7:110

Firebase SDK Version

11.2

Xcode Version

16.0

Installation Method

CocoaPods

Firebase Product(s)

Authentication

Targeted Platforms

iOS

Relevant Log Output

No response

If using Swift Package Manager, the project's Package.resolved

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If using CocoaPods, the project's Podfile.lock

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