Modernize: Fix optical misalignment in chat send button icon

[hawkticehurst]

The icon inside the chat send button (both in VS Code and Agents window) is ever so slightly not centered optically.

[vs-code-engineering]

Sharing media and context from merged PR #324634 for verification:

Optically centers the chat send button icon within the button (when hover styles are shown).

Screen.Recording.2026-07-06.at.3.18.40.PM.mov

Source PR: #324634