fix(amazonq): enable external links to open in system browser

[aseemxs]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Description

Problem: External links in Amazon Q chat (e.g., in /help command) were not clickable in JetBrains IDEs. Links would not open when clicked.

Root Cause:

1. The existing link click path (response-body-link-click) routes through handleFlareChatMessages which doesn't handle legacy commands
2. Static help text contains raw tags that bypass mynah-ui's callback system entirely
3. JCEF (embedded browser) doesn't automatically handle external link navigation like VSCode webviews do

Fix:

• Added a global document-level click handler to intercept all external link clicks (http/https)
• Added open-external-link command handler in BrowserConnector.kt to open links via BrowserUtil.browse()
• Fixed link color (was incorrectly resolving to black due to CSS variable issue)

Known Limitation: cursor: pointer is set in CSS but may not visually change on hover due to JCEF off-screen rendering mode.

Checklist

• My code follows the code style of this project
• I have added tests to cover my changes
• A short description of the change has been added to the [CHANGELOG](https://github.com/aws/aws-toolkit-jetbrains/blob/master/CONTRIBUTING.md#contributing-via-pull
  -requests) if the change is customer-facing in the IDE.
• I have added metrics for my changes (if required)

License

I confirm that my contribution is made under the terms of the Apache 2.0 license.