Change Firestore credentials to use shared_ptr

[a-maurice]

Description

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Change Firestore's Auth and App Check credentials to use shared_ptr's instead of unique_ptr. This is because the iOS repo changed to use shared_ptrs recently, and the mismatch seems to be causing runtime errors. See firebase/firebase-ios-sdk#15558

Fixes firebase/firebase-unity-sdk#1383

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Testing

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Running tests locally

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Type of Change

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• 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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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

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[github-actions]

✅  Integration test succeeded!

Requested by @a-maurice on commit 4416844
Last updated: Wed Feb 18 12:24 PST 2026
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