allozaur/feat/chat form contenteditable (#26717)

* feat: Add contenteditable tokenizer for badge/code-chip chat input

* feat: Add source-space undo/redo history for the rich input

* feat: Split text glued to a closing code fence onto its own line

* feat: Add ChatFormContenteditable rich input renderer

* feat : wire the contenteditable into ChatForm with auto-switch gating

tools/ui/tests/client/components/ChatFormContenteditableHarness.svelte

@@ -0,0 +1,27 @@
+<script lang="ts">
+	import { untrack } from 'svelte';
+	import ChatFormContenteditable from '$lib/components/app/chat/ChatForm/ChatFormContenteditable.svelte';
+
+	interface Props {
+		value?: string;
+	}
+
+	let { value: initial = '' }: Props = $props();
+
+	let value = $state(untrack(() => initial));
+	let inputRef: ChatFormContenteditable | undefined = $state(undefined);
+
+	export function getValue() {
+		return value;
+	}
+
+	export function getCaretOffset() {
+		return inputRef?.getCaretOffset();
+	}
+
+	export function setCaretOffset(offset: number) {
+		inputRef?.setCaretOffset(offset);
+	}
+</script>
+
+<ChatFormContenteditable bind:this={inputRef} bind:value />