Fix Windows DrawingView image rendering so exported/rendered strokes use the correct (round) line caps

[MFinkBK]

Description of Change

Fixes rendering final image of DrawingView that was using wrong line cap style on Windows.

Linked Issues

• Fixes [BUG] DrawingView with large LineWidth #3215

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

No tests, as it can't be tested automatically. No documentation, as it's a bugfix.

[Copilot]

Pull request overview

Fixes Windows DrawingView image rendering so exported/rendered strokes use the correct (round) line caps, addressing the visual artifacts reported in #3215 when LineWidth is large.

Changes:

• Introduced a reusable CanvasStrokeStyle configured with round start/end caps.
• Updated Win2D DrawLine calls to use the configured stroke style when rendering ink strokes.
• Added the required Win2D geometry namespace import.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[TheCodeTraveler]

Thanks @MFinkBK!

[MFinkBK]

@TheCodeTraveler You're welcome!