Sync: wire device_info writing into the login flow for the unified device list

[CDRussell]

Task/Issue URL: https://app.asana.com/1/137249556945/project/72649045549333/task/1216792641477210?focus=true
Tech Design URL (if applicable):
API Proposals URL(s) (if applicable):

Description

Adds LoginDeviceInfoWriter, invoked fire-and-forget after a successful native (ddg) login. It brings the signed-in device into the unified device list world.

Login is the point where a device that just authenticated should publish its device_info and, if needed, make the account key readable by its own credential.

Behaviour / safety
• Gated on SyncFeature.canWriteUnifiedDeviceList(), which is still off by default
• Best-effort: the legacy name/type written by the login itself is the must-succeed part; nothing here can fail or block login

Steps to test this PR

Creating account

• Fresh install internal variant
• Configure feature flags
  • canUseV2ConnectFlow - should be enabled already, just check
  • canWriteUnifiedDeviceList leave it disabled.
  • canReadUnifiedDeviceList make this enabled
• Open Sync Dev Settings and tap Create account. Copy the recovery code by tapping on it as you'll need it later
• Tap Fetch & decrypt devices and verify no device_info

Sign out / sign in

• Enable canWriteUnifiedDeviceList
• Tap Reset migration marker (just in case a migration ran)
• Sign out of sync
• Sign back in using the recovery code from earlier
• Verify you see the following logs:

     Sync-UnifiedDevices: no account_info key in login response; migration will create it
     Sync-UnifiedDevices: registering account_info key (…, wraps=1)
     Sync-UnifiedDevices: setKeysIfAbsent purpose=account_info, wraps=1
     Sync-UnifiedDevices: set-if-absent 201 — our keypair won
     Sync-UnifiedDevices: writing device_info for this device
     Sync-UnifiedDevices: patchDevices ok — 1 devices_v2 returned
     Sync-UnifiedDevices: migration complete for this device (1 devices_v2 returned)

[CDRussell]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Sync: wire device_info writing into the login flow for the unified device list #9415 👈 (View in Graphite)
• Remove gzip exclusion from /sync/devices endpoint now that backend supports it #9405
• Sync: read device_info in the device list, preferring it over legacy device data #9395
• develop

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