refactor(transcription): rename @mention test-host to test-fixture

Test-only helper colocated with production code now has a visible
.test-fixture.svelte boundary so eslint-boundaries and code search
do not confuse it for a production component. The internal alias was
also bumped from *Host to *Fixture for consistency. No behaviour
change. Felix #3 / Nora #3 on PR #629.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/MentionDropdown.test-fixture.svelte

@@ -0,0 +1,32 @@
+<script lang="ts">
+import { untrack } from 'svelte';
+import MentionDropdown from './MentionDropdown.svelte';
+import type { components } from '$lib/generated/api';
+
+type Person = components['schemas']['Person'];
+type DropdownState = {
+	items: Person[];
+	command: (item: Person) => void;
+	clientRect: (() => DOMRect | null) | null;
+};
+
+type Props = {
+	model: DropdownState;
+	initialEditorQuery: string;
+	/** Test hook: receives a setter for editorQuery so the test can mutate it. */
+	onReady?: (setEditorQuery: (q: string) => void) => void;
+	onSearch?: (q: string) => void;
+};
+
+let { model, initialEditorQuery, onReady, onSearch = () => {} }: Props = $props();
+
+let editorQuery = $state(untrack(() => initialEditorQuery));
+
+$effect(() => {
+	onReady?.((q) => {
+		editorQuery = q;
+	});
+});
+</script>
+
+<MentionDropdown model={model} editorQuery={editorQuery} onSearch={onSearch} />