fix(settings/routers): Enter adds the typed model id, never a lookalike

Typing the complete, valid id "qwen/qwen-2" and pressing Enter used to
silently add "qwen/qwen-2.5-72b-instruct" instead - the catalog row that
substring-matched the query.

WHY THIS MATTERS
This combobox is the write path for the router allowlist that the
backend now enforces at request time, so what lands in it decides which
models a user can run at all. An Enter that swaps the intended id for a
lookalike writes the wrong model into that allowlist without the user
noticing - the labels differ by a truncated suffix - and with per-token
pricing varying by orders of magnitude between such neighbors, "close"
is not good enough.

WHAT AN IMPLICIT HIGHLIGHT IS - AND WHY IT'S GONE
onChange used to point the active row at index 0 whenever anything
matched, and Enter preferred the active row. That means merely TYPING
claimed a highlight the user never asked for. Now the highlight only
ever follows an explicit gesture - ArrowDown/ArrowUp or pointer hover -
including when the list is opened from the chevron.

HOW ENTER RESOLVES NOW
1. explicit highlight → add that catalog row (unchanged);
2. no highlight + id-shaped text → add the typed id verbatim
   (normalizeTypedModelId, sharing F1's slug-preserving rule);
3. no highlight + non-id text → silent no-op, so Enter mid-search never
   errors and never adds anything.
The "Press Enter to add this model ID." hint now appears whenever the
typed text is id-shaped - previously it only showed at zero matches,
exactly the case where the trap couldn't fire.

Tests: RouterSettingsSection.test.tsx - the exact qwen scenario (typed
id added verbatim, no aria-activedescendant after typing, hint visible
alongside matches) and the Enter-no-op case. Verified by stashing the
fix: both fail on the ported code (the first adds the 72b lookalike).
Arrow-key + Enter selection keeps its existing passing test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>