Keep the integration device ID from its first read

[bgoncal]

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Summary

integrationDeviceID, the identifier the server knows an installation by, was derived from the platform's vendor identifier on every read. The first value seen is now stored in the app group and preferred from then on, so a build that derives the identifier differently keeps the registration it already has instead of registering a second device the next time a server is added.

This prepares the move from the Mac Catalyst app to the native macOS app (#5935): the native app cannot produce UIDevice.identifierForVendor, and in a test upgrade on the same Mac, adding a server again went to the "name this device" step because the server's registered device did not match. With the identifier stored by a Catalyst release first, the native app reads the same value.

Nothing is stored while the platform has no identifier to give, so a later read can still pick it up; until then the keychain deviceID fallback applies as before.

Screenshots

Link to pull request in Documentation repository

Documentation: home-assistant/companion.home-assistant#

Any other notes

Should ship in a release before the native macOS app does.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation matches the migration requirements and includes appropriate isolated tests.

Review effort: Balanced
Findings: None

What changed in this PR

Persists the first available vendor identifier to preserve server registration across Catalyst-to-native macOS migration.

Changes:

• Store and reuse the first non-nil vendor identifier.
• Add persistence and fallback tests.
• Isolate the Watch registrar test from persisted state.

File	Description
Sources/​Shared/​Settings/​SettingsStore.swift	Persists and reuses the integration device ID.
Tests/​App/​Settings/​SettingsStoreIntegrationDeviceID.test.swift	Tests persistence and delayed identifier availability.
Tests/​Shared/​Watch/​WatchDeviceRegistrar.test.swift	Preserves stored ID around the test.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.74%. Comparing base (763d6f0) to head (a45d704).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5943      +/-   ##
==========================================
+ Coverage   51.65%   51.74%   +0.09%     
==========================================
  Files        1231     1231              
  Lines       81093    81146      +53     
==========================================
+ Hits        41888    41990     +102     
+ Misses      39205    39156      -49     

Files with missing lines	Coverage Δ
Sources/Shared/Settings/SettingsStore.swift	86.80% <100.00%> (+0.18%)	⬆️

... and 8 files with indirect coverage changes

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