Improve AI chat UX (#17974)

## Summary
- update AI chat message typography and list line-height for readability
- apply richer markdown-section styling for headings, spacing,
separators, and inline code
- keep links non-underlined by default with underline on hover, using
accent11 for link color
- preserve previous AI chat table design while keeping other markdown
improvements

## Validation
- yarn eslint
packages/twenty-front/src/modules/ai/components/LazyMarkdownRenderer.tsx
packages/twenty-front/src/modules/ai/components/AIChatMessage.tsx

---------

Co-authored-by: Félix Malfait <felix.malfait@gmail.com>

packages/twenty-front/src/modules/ai/utils/isThinkingStepPart.ts

@@ -0,0 +1,14 @@
+import { isToolUIPart } from 'ai';
+import { type ExtendedUIMessagePart } from 'twenty-shared/ai';
+
+import { type ThinkingStepPart } from '@/ai/utils/thinkingStepPart';
+
+export const isThinkingStepPart = (
+  part: ExtendedUIMessagePart,
+): part is ThinkingStepPart => {
+  if (part.type === 'reasoning') {
+    return true;
+  }
+
+  return isToolUIPart(part) && part.type !== 'tool-code_interpreter';
+};