fix(auth): don't reset MFA enrollment state when already in the flow

[demolaf]

• Related to fix(auth): clear MFA enrollment flow state on every entry #2473
• Related to feat(auth)!: require the multi-step auth screens to be given navigation #2480

#2473 clears MfaEnrollmentFlowState on entry to the flow, funnelling both host entry points
through enterMfaEnrollment so no spelling of "enter the flow" skips the clear. The reset is
unconditional, and entry is reachable from a destination that stays composed while the push onto
it runs — so a second tap can arrive after the flow is already entered, and the reset that exists
to clear the previous enrolment clears the one in progress instead. onNavigate widens it: it
takes any AuthRoute, so a host can name a step of the flow while the flow is already on the
stack.

enterMfaEnrollment is now a no-op when a step of the flow is already on the stack — the flow is
already entered, so there is nothing left to do. That also enforces the pushUnique buried-case
invariant that was previously only asserted in a comment. The pop-fade case is deliberately not
guarded: the stack is already truncated by then, so entry there is a legitimate fresh one.

Adds 3 tests driving the real FirebaseAuthScreen: re-entry from inside the flow through both
host entry sites, including onNavigate naming a different step, and sign-out → second user,
which is the case the clear exists for and was pinned only at the unit level before.

This PR also reset the un-hosted MfaEnrollmentScreen after a completed enrolment. #2480 removed
the un-hosted mode outright — step, onNavigateToStep, onNavigateBack and flowState are now
required — so that defect went with the code path and those commits were dropped rather than
rebased.

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Maintainer note: Fixes internal CPRN-405

[gemini-code-assist]

Code Review

This pull request prevents state resets and duplicate entries when navigating within the MFA enrollment flow, particularly addressing issues with second taps and un-hosted screens. It introduces a guard to prevent re-entry when a flow step is already on the stack, resets the flow state on completion for un-hosted screens, and adds comprehensive tests covering these scenarios. The reviewer suggests reusing the existing mfaEnrollmentStartStep function in unhostedStartStep to reduce code duplication and improve maintainability.