Fix AppCheck concurrency infinite recursion bug in Auth

[paulb777]

Resolves #16549.

Description

This PR addresses an infinite recursion crash (yielding EXC_BREAKPOINT / SIGTRAP inside _assertionFailure) that happens in FirebaseAuth, FirebaseAI, and FirebaseFunctions when bridging AppCheckInterop into Swift concurrency.

The root cause of the crash is a known bug in Swift concurrency (particularly manifesting on older deployments or in some toolchains, like the Flutter iOS build environment) regarding generic reabstraction thunks.
When the compiler attempts to automatically synthesize the async counterpart for the Objective-C protocol method getToken(forcingRefresh:completion:), or when the return type of a Task / withCheckedContinuation generic contains the protocol existential FIRAppCheckTokenResultInterop?, it produces a faulty reabstraction thunk (partial apply forwarder for generic not re-abstracted specialization <__C.FIRAppCheckTokenResultInterop?>...). This thunk infinitely loops on itself when the continuation resumes.

Because the old code invoked the auto-bridged await appCheck.getToken(forcingRefresh:) directly, this buggy generic thunk was inevitably linked into the local Task or async state machine.

The Fix

To completely avoid the compiler bug, we must prevent the existential protocol object from ever crossing an async/await boundary.

This PR updates the internal implementations across FirebaseAuth, FirebaseAI, and FirebaseFunctions to use withCheckedContinuation wrapping the raw block-based getToken(forcingRefresh:completion:) selector. Inside the completion block, we extract the concrete native Swift types (token as String and error as Error?) and resume the continuation with a tuple (String, Error?) (or just String?).

Since String and Error? are native concrete Sendable types, the async state machine is fully decoupled from the Objective-C protocol existential, bypassing the compiler's buggy thunk generation entirely.

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[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request addresses a persistent crash when interacting with App Check under certain Swift toolchains (such as the Flutter SDK) by avoiding a compiler bug in Swift concurrency thunk generation. The changes replace direct async calls to getToken with manual bridging using withCheckedContinuation and callback-based methods, returning a tuple (token: String, error: Error?) instead of FIRAppCheckTokenResultInterop. Feedback suggests simplifying the tuple construction in AppCheckInterop+FirebaseAuth.swift by directly passing the properties of result to continuation.resume(returning:) without intermediate local variables.

[paulb777]

/gemini review

[gemini-code-assist]

Code Review

This pull request addresses a persistent crash when interacting with App Check under certain Swift toolchains (such as the Flutter SDK) caused by a compiler bug in Swift concurrency thunk generation. It achieves this by replacing direct calls to the async getToken method with a manual wrapper using withCheckedContinuation over the closure-based getToken method, returning a tuple (token: String, error: Error?) instead of FIRAppCheckTokenResultInterop. These updates are applied across FirebaseAI, FirebaseAuth, and FirebaseFunctions. There are no review comments to address.