Make links clickable in Windows Companion chat

Problem URLs and Markdown links displayed in Windows Companion chat are rendered as text rather than clickable links. Users must manually copy and paste URLs into a browser, which makes documentation, issue references, tool output, and assistant responses harder to use. ## Expected behavior - Plain URLs such as https://example.com are detected and rendered as clickable links. - Markdown links such as [Example](https://example.com) are rendered with clickable link text. - Activating a link opens it using the user's default browser. - Link text remains selectable and supports normal keyboard navigation and accessibility behavior. - Unsupported or malformed URI schemes are not launched. ## Acceptance criteria - Links are clickable in assistant and user messages. - Links remain clickable in bulleted lists, tables, and other supported Markdown content. - Mouse, keyboard, and screen-reader activation are supported. - Tests cover plain URLs, Markdown links, malformed links, and unsafe URI schemes.

Screenshot

The authorization URL is rendered as plain text and cannot be activated directly from chat. The one-time code has been redacted.

Start at the Windows Companion chat message-rendering entry point and locate the existing Markdown rendering and message tests. Trace how assistant and user messages handle URLs, then verify completion with coverage for plain and Markdown links, supported content such as lists and tables, keyboard and screen-reader activation, malformed links, and unsafe URI schemes.