Add Recaptcha App Check Provider stubs

[a-maurice]

Description

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

Add stub implementations for a new built-in provider, Recaptcha Enterprise. This is stubbed on all platforms, and will eventually be implemented for both Android and iOS.

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Testing

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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.

[gemini-code-assist]

Code Review

This pull request introduces the RecaptchaEnterpriseProviderFactory for App Check, adding the header and stub implementation files and updating CMakeLists.txt. The review feedback highlights a critical path error in CMakeLists.txt where the header path is missing the 'app_check/' subdirectory, and recommends initializing the 'internal_' member pointer to nullptr in the constructor to prevent undefined behavior.

[gemini-code-assist]

The path to the header file is missing the app_check/ subdirectory. It should be src/include/firebase/app_check/recaptcha_enterprise_provider.h to match the actual file location and avoid build failures.

  src/include/firebase/app_check/recaptcha_enterprise_provider.h

[gemini-code-assist]

The member variable internal_ is left uninitialized in the constructor. It is best practice to initialize member pointers to nullptr to prevent undefined behavior or garbage values.

Suggested change

	RecaptchaEnterpriseProviderFactory::RecaptchaEnterpriseProviderFactory() {}
	RecaptchaEnterpriseProviderFactory::RecaptchaEnterpriseProviderFactory() : internal_(nullptr) {}