refactor(chat): drop the inert content-change and animation-handler contract

The old scroll engine needed message parts to report content growth
(onContentChange) and per-message animation lifecycle callbacks
(AnimationHandlers) so it could re-pin the viewport. The timeline list
measures growth itself now, and the replacement hook had already stubbed
the whole contract with no-ops kept only for source compatibility.

Remove it end to end: the hook exports, the container and list threading,
the ChatMessage/MessageBody signal-only effects, and every part-level
prop and call site. Expand/collapse behavior and reveal animations are
untouched — only the reporting channel goes.

packages/ui/src/components/chat/message/parts/AssistantTextPart.tsx

@@ -2,7 +2,6 @@ import React from 'react';
 import type { Part } from '@opencode-ai/sdk/v2';
 import { MarkdownRenderer } from '../../MarkdownRenderer';
 import type { StreamPhase, ToolPopupContent } from '../types';
-import type { ContentChangeReason } from '@/hooks/useChatTimelineScroll';
 import { useStreamingTextThrottle } from '../../hooks/useStreamingTextThrottle';
 import { resolveAssistantDisplayText, shouldRenderAssistantText } from './assistantTextVisibility';
 import { streamPerfCount, streamPerfObserve } from '@/stores/utils/streamDebug';
@@ -17,7 +16,6 @@ interface AssistantTextPartProps {
     messageId: string;
     streamPhase: StreamPhase;
     chatRenderMode?: 'sorted' | 'live';
-    onContentChange?: (reason?: ContentChangeReason, messageId?: string) => void;
     onShowPopup?: (content: ToolPopupContent) => void;
 }