[Auth] Reduce callback nesting

[ncooke3]

Potential fix for #14962. The stack trace looks like the interop's completion handler is released twice:

Crashed: com.google.firebase.auth.globalWorkQueue
0  libobjc.A.dylib                0x1b50 objc_release_x0 + 16
1  libobjc.A.dylib                0x1b50 objc_release + 16
2  Porter                         0xd61c2c closure #1 in Auth.getToken(forcingRefresh:completion:) + 1594 (User.swift:1594)
3  Porter                         0x14880 thunk for @escaping @callee_guaranteed @Sendable () -> () + 4306913408 (<compiler-generated>:4306913408)
4  libdispatch.dylib              0x1aac _dispatch_call_block_and_release + 32
5  libdispatch.dylib              0x1b584 _dispatch_client_callout + 16
6  libdispatch.dylib              0xa2d0 _dispatch_lane_serial_drain + 740
7  libdispatch.dylib              0xadac _dispatch_lane_invoke + 388
8  libdispatch.dylib              0x151dc _dispatch_root_queue_drain_deferred_wlh + 292
9  libdispatch.dylib              0x14a60 _dispatch_workloop_worker_thread + 540
10 libsystem_pthread.dylib        0x4660 _pthread_wqthread + 292
11 libsystem_pthread.dylib        0x19f8 start_wqthread + 8

The Auth interop's Swift implementation is being passed a completion from an Objective-C++ context (Firestore). My hunch is that Swift implementation's wrapper is confusing the memory management in the handoff of the ObjC impl. callback to the Swift Task scope.

Here is the difference in the call stack:

- Task -> wrapper closure -> DispatchQueue.main.async -> original callback
+ Task  -> DispatchQueue.main.async -> original callback

Fixes #14962

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