[Testing] Improve SettingsManager tests — round-trip, encryption assertion, error handling

[jackby03]

Summary

The SettingsManager tests exist but are incomplete — they test serialization in isolation and bypass the file system. There is no test covering the full save → load round-trip, and the ICryptographyService mock does not assert that keys are actually encrypted before being written to disk.

What needs to be done

• Full round-trip test: Save(settings) → read file → Load() → assert values match
• Verify encrypted file: file on disk must not contain plain-text API key string
• Verify that Load() on a missing file returns default settings (no exception)
• Verify that Load() on a corrupted file returns default settings and logs an error
• Verify that Save() uses ICryptographyService.Encrypt for credential fields
• Test ExchangeCredential add/remove/set-active flows

Acceptance criteria

• Round-trip test passes with temp directory
• Corrupted file test passes (no crash)
• Raw key is never written in plain text
• All credential CRUD operations are covered
• Tests clean up temp files in Dispose

References

• src/Omnijure.Core/Features/Settings/
• tests/Omnijure.Core.Tests/SettingsManagerTests.cs