Support Auth requests when Appcheck is enabled for windows applications

[lalmstrom]

Description

Fixes Auth requests when Appcheck is enabled for desktop applications. Without it, Auth requests are rejected.

Adds X-Firebase-AppCheck with token header to Authorization requests.

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Testing

Have tested Auth requests in flutter application running in windows with debug tokens in firebase.

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Type of Change

• 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 integrates AppCheck attestation tokens into the AuthRequest constructor by fetching them via the FunctionRegistry and adding them to the request headers. Feedback suggests improving null safety for the function registry, adding more robust error and empty-token checks, and warns about the blocking nature of the Await call which could impact the UI thread if not handled in an asynchronous context.

[a-maurice]

Hi @lalmstrom, thanks for making this PR. Generally it all looks good, but we'll need some time to test it out, since this is all rather finicky, and I want to avoid any problems with Auth since it is so core.

[lalmstrom]

Hi @lalmstrom, thanks for making this PR. Generally it all looks good, but we'll need some time to test it out, since this is all rather finicky, and I want to avoid any problems with Auth since it is so core.

Understood. We are blocked by this fix. Have not found a simple workaround to use this fix in flutter, so I am waiting for a release of the SDK with this fix to get flutter packages to be updated to a newer release.

Note: A similar fix is needed for cloud functions as well, but we currently have a workaround using REST calls for now.