Fix CFI icall violation in Future callbacks

[a-maurice]

Description

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

Eliminate unsafe reinterpret_cast of TypedCompletionCallback and std::function completion callbacks to FutureBase::CompletionCallback in Future::OnCompletion and Future::AddOnCompletion.

Introduce type-safe trampoline functions and wrapper data structures (TypedCompletionCallbackTrampoline and TypedCompletionCallbackData) in firebase::detail to bridge the typed callback to the base callback signature expected by ReferenceCountedFutureImpl::RunCallback, avoiding undefined behavior and runtime CFI type check aborts.

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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.

[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit 79c2210
Last updated: Mon Aug 31 20:10 PDT 2026
View integration test log & download artifacts

Failures	Configs
missing_log	[TEST] [ERROR] [Android] [All 3 os] [1/2 android_device: android_target]
functions	[TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   CRASH/TIMEOUT
installations	[TEST] [FAILURE] [iOS] [macos] [1/2 ios_device: simulator_target] (1 failed tests)   FirebaseInstallationsTest.TestCanGetToken

Add flaky tests to go/fpl-cpp-flake-tracker

[gemini-code-assist]

Code Review

This pull request refactors the Future<T>::OnCompletion and Future<T>::AddOnCompletion implementations to avoid unsafe reinterpret_cast on callbacks, introducing a typed callback data structure and trampoline functions instead. The review feedback highlights critical safety issues regarding null or empty callbacks: passing a null callback can cause a crash if the future is already completed, and wrapping an empty std::function without validation can trigger a std::bad_function_call exception or crash. Suggestions are provided to handle these cases safely by using dummy callbacks or adding validity checks.