Fix cancellation logic in Picker onDidChangeValue handler (fixes #247945)

[pelmers-db]

The onDidChangeValue in the quick access picker has logic to cancel the previous search as the user is typing.

However this logic was never actually called because the value setter on DisposableStore called dispose() on the previous value. The problem is that calling dispose() without cancel() just removes all the listeners without triggering their cancellation handlers. So the dispose(true) runs but it's a no-op; there's nothing left to cancel.

This PR fixes the issue by running the cancellation first, then updating the DisposableStore value.

I observe a 2-10x faster experience when typing in a fairly large workspace, depending on the exact search term.

Resolves #247945

[TylerLeonhardt]

Nice find! LGTM