[App] Add link.xml to preserve Auth types for reflection

[AustinBenoit]

Description

Provide details of the change, and generalize the change in the PR title above.
Add a link.xml file to the core App module in app/src/internal/. This file prevents Unity's IL2CPP compiler from aggressively stripping Firebase.Auth types (FirebaseAuth and FirebaseUser) when they are accessed via reflection by other modules like Functions or Firebase AI (via FirebaseInterops.cs).
This resolves runtime INTERNAL errors on iOS/tvOS where the Functions SDK failed to retrieve Auth tokens due to missing symbols, resulting in backend assertion failures.

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Testing

Describe how you've tested these changes.

• Verified that the tokenTest in the Functions integration test app passes on the iOS simulator after adding link.xml.
• Verified that link.xml is correctly bundled in both .unitypackage and UPM (.tgz) generated artifacts via the build scripts.
• Full run of tests verifying that they all pass

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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[gemini-code-assist]

Code Review

This pull request introduces a link.xml configuration file to ensure that FirebaseAuth and FirebaseUser types are preserved during the linking process. The reviewer suggested also including the FirebaseAppCheck type to prevent potential runtime stripping issues, as reflection is also performed on that assembly in the codebase.

[github-actions]

❌  Integration test FAILED

Requested by @firebase-workflow-trigger[bot] on commit dcf7d02
Last updated: Thu Apr 16 12:22 PDT 2026
View integration test log & download artifacts

Failures	Configs
D:\a\firebase-unity-sdk\firebase-unity-sdk\testapps\Unity2021.3.43f1-NET4.6\FirebaseAI\testapp	[TEST] [ERROR] [2021] [1/2 Build OS(s): windows] [1/6 Platform(s): Playmode] [1/8 Test Device(s): github_runner]
firestore	[TEST] [ERROR] [2021] [1/2 Build OS(s): windows] [1/6 Platform(s): Android] [1/8 Test Device(s): android_target]
remote_config	[BUILD] [ERROR] [2021] [1/2 Build OS(s): macos] [1/3 Platform(s): 14] [2/4 Test Device(s): iOS tvOS]
storage	[BUILD] [ERROR] [2021] [1/2 Build OS(s): macos] [1/3 Platform(s): 14] [2/4 Test Device(s): iOS tvOS] [TEST] [ERROR] [2021] [1/2 Build OS(s): windows] [1/6 Platform(s): Playmode] [1/8 Test Device(s): github_runner] [TEST] [FAILURE] [2021] [1/2 Build OS(s): macos] [1/6 Platform(s): 14] [2/8 Test Device(s): Windows macOS]