Document supportsUnifiedCompletions startup-flicker caveat

Add a code comment at the reactive read site explaining the brief
cold-start window on the fetched /models path where the separate
completions provider is not yet excluded. Matches the Known limitations
section in the PR description.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 676b2f6f-4b80-4754-8dde-708d7e992148