Fix listener leak: move onDidChangeConfiguration out of onDidProgressStep callback

[SebTardif]

Problem

In GettingStartedPage, a configurationService.onDidChangeConfiguration listener for REDUCED_MOTION_KEY was registered inside the onDidProgressStep event callback using this._register(). Each time a walkthrough step was completed, a new permanent config listener accumulated in the disposable store and was never cleaned up.

After completing N walkthrough steps, N identical listeners existed, each toggling the same CSS class on config changes. This is the exact anti-pattern the project's coding guidelines warn about:

Do NOT register a disposable to the containing class if the object is created within a method that is called repeatedly to avoid leaks.

Fix

Move the onDidChangeConfiguration subscription to the constructor, alongside the other config/event listeners, so it is registered exactly once for the lifetime of the page.

Fixes #315168

[Copilot]

Pull request overview

Fixes a disposable/listener leak in the Getting Started editor by ensuring the REDUCED_MOTION_KEY configuration listener is registered exactly once for the lifetime of GettingStartedPage, rather than once per walkthrough step progress event.

Changes:

• Move configurationService.onDidChangeConfiguration (for REDUCED_MOTION_KEY) out of the onDidProgressStep callback.
• Register that configuration listener in the constructor alongside other long-lived listeners.

[bhavyaus]

Thanks!