test(auth): cover the write-through recoveries and the custom reauth slot's sheet chrome

[demolaf]

Three error recoveries hand the step they navigate to a value the failure carried, and none were covered: AccountLinkingRequiredException writes pendingLinkingCredential, and EmailLinkPromptForEmailException / EmailLinkCrossDeviceLinkingException both write emailLinkFromDifferentDevice. Separately, a custom reauthContent slot is Bare only on MethodPicker — email and phone sub-steps still wrap in the library sheet, and nothing asserted that chrome was present.

• FirebaseAuthScreenEmailRecoveryTest: 7 cases covering landing step and address for each of the three exceptions, plus the write-through itself — a real linkWithCredential verification for account linking, and isSignInWithEmailLink for both email-link failures.
• ReauthSurfaceGateTest: 3 cases — the method picker composes bare under a custom slot, and the email and phone steps are asserted to be inside the sheet via ancestry rather than as separate presence checks.

Checked each case against a mutated production branch: dropping the credential write, nulling the prompt's link, swapping the cross-device branch to providerName, and ignoring the step in the reauth gate each fail only their own test.

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Maintainer note: Fixes internal CPRN-404

[gemini-code-assist]

Code Review

This pull request adds comprehensive unit tests to FirebaseAuthScreenEmailRecoveryTest.kt and ReauthSurfaceGateTest.kt. The new tests in FirebaseAuthScreenEmailRecoveryTest.kt verify account recovery flows that pass data (such as credentials or links) to subsequent steps, including account linking, email-link prompts, and cross-device linking. The tests added to ReauthSurfaceGateTest.kt validate the behavior of custom-slot presentation gates, ensuring that the method picker is rendered bare while sub-flows are properly enclosed within the library's sheet. There are no review comments, and I have no feedback to provide.