fix(auth): show the link-sent confirmation on anonymous upgrade, with e2e tests

[demolaf]

Sending an email link while an anonymous user was signed in showed no confirmation. Retracting the EmailSignInLinkSent notification to Idle makes authStateFlow()'s combine fall back to reporting the session, so the pre-existing anonymous session arrived as a fresh Success and reset the back stack to the success route, tearing the dialog down with the screen that owned it.

• FirebaseAuthScreen.kt: the steps that own a "link sent" confirmation are no longer reset off by a re-asserted session — only a real credential exchange, which is the only thing carrying an AuthResult, still completes to the success route. Applied to the RequiresEmailVerification / RequiresProfileCompletion branch too, since authUserState routes an unverified password user there rather than to Success. The guard is stateless, so it holds across activity recreation and is unaffected by StateFlow conflation. Same reasoning as the existing Phone.EnterPhoneNumber exemption in the Idle branch.

Email link sign-in had a single e2e test buried in EmailAuthScreenTest. It now has its own EmailLinkAuthScreenTest with nine, covering the cross-device prompt and completion, the method-picker hand-off, anonymous upgrade, and the wrong-device, foreign-anonymous-id and different-anonymous-user guards — with the anonymous upgrade case asserting the flow stayed on the email link step, verified to fail on the old code and pass with the fix. Link delivery now fails rather than Assume-skips on timeout, so a slow emulator can no longer report the class green. 966 unit and 71 e2e tests pass.

This is an interim fix. The underlying cause — one-shot outcomes modelled as AuthState values, which is what makes retraction expose the session at all — is tracked separately and would delete this guard along with five other workarounds.

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Maintainer note: Fixes internal CPRN-422

[gemini-code-assist]

Code Review

This pull request refactors the email link sign-in logic and significantly expands its test coverage. In FirebaseAuthScreen.kt, a check is added to prevent resetting the backstack to AuthRoute.Success on notification trailing events. Additionally, email link sign-in tests are extracted from EmailAuthScreenTest.kt into a dedicated EmailLinkAuthScreenTest.kt suite with comprehensive end-to-end scenarios. Feedback includes addressing a potential infinite loop in Robolectric tests due to virtual clock behavior when using Thread.sleep() with System.currentTimeMillis(), and safely checking if lateinit properties are initialized in tearDown() to avoid masking setup failures.