Auth - avoid repeated account lookups

[xingsy97]

Problem

AuthenticationService.getAccounts() queries all sessions via getSessions() on every call without caching, as noted by a TODO comment from @TylerLeonhardt (// TODO: Cache this).

Fix

Add a per-provider Map<string, ReadonlyArray<AuthenticationSessionAccount>> cache that is populated on first call and invalidated when:

• The provider fires onDidChangeSessions (cache is cleared before the event is re-fired to consumers, ensuring any consumer calling getAccounts() in the event handler sees fresh data)
• Extension session access changes for the provider (onDidChangeExtensionSessionAccess)
• The provider is unregistered (unregisterAuthenticationProvider)

Validation

• Cache-miss path is identical to the original code
• Cache invalidation covers all mutation paths: session changes, access changes, and provider removal
• Invalidation ordering ensures consumers always see consistent data in event callbacks
• No dispose cleanup needed: cache holds pure data objects, GC handles it when the service is disposed

[Copilot]

Pull request overview

Adds an in-memory, per-auth-provider cache for AuthenticationService.getAccounts() to avoid re-querying all sessions on every call, with cache invalidation wired into existing session/access lifecycle events.

Changes:

• Introduces _accountsCache keyed by provider id and populates it on first getAccounts() call.
• Invalidates cached entries when sessions change, when extension session access changes, and when a provider is unregistered.