Add SyncAutoRestore API and SyncRecovery key

[CDRussell]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1213488933349586/task/1213232213875729?focus=true

Description

Adds the SyncAutoRestore API and internal dev tooling for validating Block Store integration.

What's in this PR (stacked on persistent-storage module)

• SyncAutoRestore interface in sync-api with canRestore() and restoreSyncAccount()
• RealSyncAutoRestore implementation, gated behind syncAutoRestore feature flag (defaults FALSE)
• SyncRecoveryPersistentStorageKey — the key that will store the sync recovery code in Block Store (shouldBackupToCloud = true)
• Internal settings Block Store section showing: feature flag status, availability, E2E encryption support, current stored value, write/clear controls
• Unit tests for RealSyncAutoRestore

API proposal: https://app.asana.com/1/137249556945/project/1202552961248957/task/1213554376592121

Steps to test this PR

QA optional

Block Store dev tooling

• Install internalRelease on a device with Google Play Services
• Open App Settings → Internal Settings → Sync Internal Settings
• Verify Block Store section shows: feature flag status (disabled), availability, E2E status, current value "(key not set)"
• Enter text in input → tap Write → confirm toast "Stored successfully" and value updates
• Tap Clear → confirm toast "Cleared successfully" and value returns to "(key not set)"

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Note

Medium Risk
Introduces a new cloud-backup persistent storage key for sync recovery data and wires it into a new auto-restore capability check; misconfiguration could impact sensitive data handling. Functional impact is limited by a default-off feature flag and restore being unimplemented.

Overview
Adds a new SyncAutoRestore API plus RealSyncAutoRestore implementation, gated behind a new syncAutoRestore feature flag (default off), where canRestore() checks for a stored recovery token in persistent storage and restoreSyncAccount() is currently a stub.

Introduces SyncRecoveryPersistentStorageKey (configured with shouldBackupToCloud = true) and expands Sync Internal Settings with a Block Store section to display flag/availability/E2E support and to read/write/clear the stored value for validation. Includes unit tests covering canRestore() behavior under flag and storage success/failure conditions.

^{Written by Cursor Bugbot for commit e112155. Configure here.}

[CDRussell]

• Update sync settings screen with new recovery section #8008
• Add new sync screen for showing recovery code #7966
• Prevent orphaned sync devices when auto-restoring #7969
• Headless sync recovery implementation #7941
• Sync auto recovery detection in onboarding #7925
• Add SyncAutoRestore API and SyncRecovery key #7910 👈 (View in Graphite)
• develop

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