refactor(auth): scope phone verification loading state to the composition that owns it

[demolaf]

FirebaseAuthUI carried a revision counter plus clearLoadingState and currentAuthStateRevision purely so verifyPhoneNumber could retract its own AuthState.Loading on cancellation without clobbering whatever superseded it.

verifyPhoneNumber now writes no state on cancellation and just rethrows, and PhoneAuthScreen retracts its own Loading instead — in onChangeNumberClick, and from a DisposableEffect when the composition goes away mid-verification. Cancelling an attempt was already the screen's own bookkeeping, so the state it leaves behind belongs to the screen too.

Dropped three tests covering the removed API and the old callee-retracts contract, and added one to PhoneAuthScreenVerificationLifecycleTest for the change-number case — verified it fails without the change.

[gemini-code-assist]

Code Review

This pull request simplifies state management in FirebaseAuthUI by removing the AtomicLong revision tracking and instead handling the retraction of the Loading state directly within the UI layer using Compose's lifecycle. Specifically, a DisposableEffect is introduced in PhoneAuthScreen to reset the authentication state to Idle upon disposal if it was left in a Loading state. The review feedback points out a potential bug where rememberUpdatedState(authState) combined with DisposableEffect(authUI) can cause a mismatched state capture when authUI changes, as well as a race condition with cancellation. A code suggestion is provided to track the state per authUI instance using remember(authUI) { mutableStateOf(authState) }.