[Follow-up] Simplify chat preference initialization

[Copilot]

Summary

This follow-up stacks on PR #452 by targeting jdneo-preferences-01-nonblocking-loading.

• Preference restoration remains asynchronous. A normal restore and a genuine first-run missing file use saved/default preferences without a notification.
• Restore errors are logged with the original exception, then initialization falls back once to local in-memory defaults. Fallback readiness is independent of notification dismissal.
• Recovered defaults are never persisted over unreadable, malformed, or temporarily inaccessible preference data, including during preference-service disposal or model updates.
• Preference loading/manual retry UI is replaced by a dismissible final result. Failed fallback keeps dependent controls unavailable. Mode discovery retains its independent retry and does not reset preference restoration.
• Tests cover fallback, missing files, persistence protection, coalescing, timeouts/stale results, account lifecycle, readiness, dismissal, and mode discovery.

Validation

• ./mvnw -pl com.microsoft.copilot.eclipse.ui,com.microsoft.copilot.eclipse.ui.test checkstyle:check — passed; 0 Checkstyle violations.
• git diff --check — passed.
• Secret scan — no secrets detected.
• Parallel validation — CodeQL found 0 alerts. No code-review findings; the review executable was unavailable in the environment.
• ./mvnw -pl com.microsoft.copilot.eclipse.ui.test -am test — blocked before compilation/tests while resolving base.target; https://download.eclipse.org/wildwebdeveloper/releases/1.3.2/ failed with UnresolvedAddressException.
• ./mvnw clean verify — Checkstyle passed, then the same target-platform resolution failure stopped the build.