[PM-40262] Complete Device Auth Key core support

[2001Y]

Summary

• complete the iOS Device Auth Key service against the current Bitwarden SDK
• keep Account Security UI changes out of this PR to avoid overlap with [PM-33981] feat: Add device management UI and Account Security integration #2871
• preserve account isolation and fail closed for mismatched credentials

Current state

This draft currently contains the first TDD specification only. The expected RED is DeviceAuthKeyError.notImplemented; implementation commits will follow after that behavior is confirmed in CI.

This continues the core-service direction explored in #2423 without cherry-picking its conflicting branch.

Test plan

• RED: missing-record assertion test fails on the current stub
• GREEN: targeted DeviceAuthKeyServiceTests pass
• related keychain/autofill tests pass
• BitwardenShared generic simulator build passes

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[bitwarden-bot]

Thank you for your contribution! We've added this to our internal Community PR board for review.
ID: PM-40262
Link: https://bitwarden.atlassian.net/browse/PM-40262

Details on our contribution process can be found here: https://contributing.bitwarden.com/contributing/pull-requests/community-pr-process.

[2001Y]

Closing this premature test-only draft while coordinating with the existing Device Auth Key work in #2423. Local implementation and validation will continue before any new public PR.