Update app check debug provider to match other firebase sdks

[AustinBenoit]

To alight with other SDK's the cpp app check debug provider will create a token on behalf of the user if they have not already created and registered one. Users will be prompted to register token

Description

Provide details of the change, and generalize the change in the PR title above.

Update app check debug provider to match other firebase sdks

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Ran a test ensuring the new phrase is output

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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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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.

[gemini-code-assist]

Code Review

This pull request updates the desktop debug provider for App Check to automatically generate a UUID-based debug token if the APP_CHECK_DEBUG_TOKEN environment variable is not provided, and logs a warning message with registration instructions. Feedback points out a critical compilation error due to an incomplete declaration of GenerateDebugToken and suggests a minor optimization to avoid calling std::getenv twice.

[AustinBenoit]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates the App Check Desktop Debug Provider to automatically generate a local debug token using a UUID if the 'APP_CHECK_DEBUG_TOKEN' environment variable is not set, and logs instructions for registering it. The review feedback correctly identifies a potential data race in 'GetTokenInternal' when lazy-initializing the debug token and modifying the static logging flag concurrently, and suggests using a 'std::mutex' along with including the '' header to ensure thread safety.

[a-maurice]

Unlike the other platforms, won't this generate a different debug token each time the app is run? I feel like this is more likely to cause confusion, cause the token we are telling them to use will change each time. I'd prefer if either it saved the generated token in someway and used it each time (which is admittedly going to be complicated), or to avoid doing this.