Prevent sign-out from triggering sign-in dialogs across windows

[Copilot]

Signing out in the Agents window could make each Editor window steal focus with a sign-in dialog. In-flight API failures triggered re-authentication before cached authentication state reflected the sign-out.

• Session validation: Silently check for an existing session and recheck authentication state before forcing sign-in.
• Prompt handling: Keep validation within per-provider deduplication, preserve the cooldown, and emit prompt telemetry only after validation.

• Fixes Sign-out tiggers sign-in request on each window #8951

[azure-pipelines]

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