Fix XTerm.NET build warnings

[jchristn]

Summary

This fixes the two current XTerm.NET build warnings:

• removes the unused EscapeSequenceParser.Dcs event, since DCS sequences are currently consumed by the parser state machine but not emitted to subscribers
• makes Terminal.HyperlinkChanged a real raised event from OSC 8 hyperlink handling

The hyperlink event now fires when a hyperlink starts and when it is cleared. The event args URL is nullable so the clear event can report null, matching Terminal.CurrentHyperlink.

Validation

• dotnet build src\XTerm.NET.slnx -c Release
  • 0 warnings, 0 errors
• dotnet test src\XTerm.NET.slnx
  • 584 passed, 0 failed

[Copilot]

Pull request overview

This PR targets two build warnings in XTerm.NET by removing an unused parser event and ensuring OSC 8 hyperlink handling actually raises the Terminal.HyperlinkChanged event.

Changes:

• Remove the unused EscapeSequenceParser.Dcs event.
• Add an internal Terminal.RaiseHyperlinkChanged(...) helper and invoke it from OSC 8 hyperlink start/clear handling.
• Update HyperlinkEventArgs.Url nullability and extend OSC hyperlink tests to validate event behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/XTerm.NET/Terminal.cs	Adds RaiseHyperlinkChanged(string?) to centralize raising the hyperlink event.
src/XTerm.NET/Parser/EscapeSequenceParser.cs	Removes the unused public Dcs event that was never raised.
src/XTerm.NET/InputHandler.cs	Raises hyperlink-changed notifications when OSC 8 links start and when they are cleared.
src/XTerm.NET/Events/TerminalEvents.cs	Updates HyperlinkEventArgs.Url to nullable and adjusts constructor accordingly.
src/XTerm.NET.Tests/OscSequenceTests.cs	Adds assertions that HyperlinkChanged fires for start and clear cases.

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