Use MarkdownString for privacy footer localization with embedded links

[Copilot]

Consolidates fragmented localization strings in the agents welcome page privacy footer. Previously split across multiple localize() calls and DOM nodes, making proper translation difficult.

Changes

• Replace manual anchor element creation with MarkdownString and markdown link syntax [Terms]({1}) and [Privacy Statement]({2})
• Add IMarkdownRendererService injection to render the markdown
• Use single localize() call with URL placeholders

Before:

desc.textContent = localize('tosDescription', "By continuing, you agree to {0}'s ", name);
const termsLink = append(desc, $<HTMLAnchorElement>('a'));
termsLink.textContent = localize('terms', "Terms");
desc.appendChild(document.createTextNode(localize('and', " and ")));
// ... more fragmented DOM manipulation

After:

const markdown = new MarkdownString(
  localize('tosDescription', 
    "By continuing, you agree to {0}'s [Terms]({1}) and [Privacy Statement]({2}).",
    name, termsUrl, privacyUrl
  ),
  { isTrusted: true }
);
desc.appendChild(markdownRendererService.render(markdown).element);

Follows the pattern used in chatWidget.ts, inlineChatWidget.ts, and gettingStartedContent.ts.

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