Fix relative markdown link handling

[YourRobotOverlord]

Summary

• Treat relative Markdown link targets as clickable links alongside anchors and safe absolute URIs
• Restrict OSC8 hyperlink emission to safe absolute URI schemes
• Add regression tests for relative links in Markdown text and table cells
• Add a relative-link example to the Markdown demo content

Fixes #5504

Testing

• dotnet build Terminal.Gui/Terminal.Gui.csproj --no-restore
• dotnet test --project Tests/UnitTestsParallelizable --no-restore --filter-class "*MarkdownViewTests" /p:BuildProjectReferences=false

[Copilot]

Pull request overview

Updates the Markdown rendering/click handling pipeline so relative Markdown link targets behave like other clickable links (styling + LinkClicked), while keeping OSC8 hyperlink emission restricted to safe absolute URI schemes.

Changes:

• Treat relative link targets as Markdown “link targets” for styling/clickability (underline + click events).
• Gate OSC8 hyperlink emission behind a “safe absolute URI” check (scheme allowlist).
• Add regression tests for clicking relative links in both normal Markdown text and table cells, and adjust styling expectations.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Tests/UnitTestsParallelizable/Views/Markdown/MarkdownViewTests.cs	Adds click regression tests for relative links (plain + table) and updates expected underline styling for relative links.
Terminal.Gui/Views/Markdown/MarkdownView.Drawing.cs	Restricts OSC8 emission (Driver.CurrentUrl) to safe absolute URI targets only.
Terminal.Gui/Views/Markdown/MarkdownTable.cs	Applies the same safe-absolute check for table rendering behavior tied to absolute URLs.
Terminal.Gui/Views/Markdown/Markdown.cs	Adds a relative-link example to demo Markdown content.
Terminal.Gui/Drawing/Markdown/MarkdownAttributeHelper.cs	Centralizes link-target detection (anchors/safe absolute/relative) and adds helper for safe absolute URI parsing.