Add chat markdown plain text fast path

[roblourens]

Summary:

• Add a chat markdown renderer fast path for plain text content with no markdown-significant syntax
• Preserve the expected rendered-markdown/p DOM shape while avoiding marked and sanitizer work
• Add focused tests for fast-path shape and target reuse

Validation:

• npm run compile-check-ts-native -- --pretty false
• ./scripts/test.sh --run src/vs/workbench/contrib/chat/test/browser/widget/chatMarkdownRenderer.test.ts
• node --experimental-strip-types build/hygiene.ts src/vs/workbench/contrib/chat/browser/widget/chatContentMarkdownRenderer.ts src/vs/workbench/contrib/chat/test/browser/widget/chatMarkdownRenderer.test.ts

[Copilot]

Pull request overview

Adds a fast path to ChatContentMarkdownRenderer.render that bypasses marked and the sanitizer when the input has no markdown-significant syntax, producing the same <div class="rendered-markdown"><p>…</p></div> shape directly.

Changes:

• New renderPlainTextMarkdown helper that returns a rendered-markdown element when the value matches none of three exclusion regexes (inline syntax chars, GFM autolinks, block-level syntax).
• render() short-circuits to the fast path when applicable; otherwise falls back to the existing pipeline.
• Two new unit tests cover the fast path's DOM shape and target-element reuse.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/widget/chatContentMarkdownRenderer.ts	Adds plain-text fast path with regex guards and a target-truncation step.
src/vs/workbench/contrib/chat/test/browser/widget/chatMarkdownRenderer.test.ts	Adds tests verifying fast-path DOM shape and target reuse.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1