refactor: polish quick chat decorations naming

Description

This MR improves variable names introduced in refactor(QuickChat): split decoration to hint a... (!2553 - merged) and feat: create quick chat hint when highlighting ... (!2550 - merged)

It only renames variables/methods and simplifies the gutter icon test setup. It doesn't change the business logic.

Related Issues

Part of [VS Code] QuickChat keybinding hint strange beh... (#1729 - closed)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes