Ensure that old values aren't cached inappropriately

[colin-grant-work]

What it does

Fixes #10959
Fixes #10865

Previously, the caching mechanism in ValidatedPreferenceProxy could mistakenly retain cached values for language overrides. This PR ensures that the cache is maintained only once the proxy subscribes to preference events and that it is updated for all overrides in case of a preference change.

How to test

1. Activate and deactivate the editor.minimap.enabled preference. It should immediately take effect in open editors (in all languages) and apply correctly to new editors.
2. Assert that the new tests pass.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

@colin-grant-work I believe this fixes #10959 and #10865?

[colin-grant-work]

@msujew, thanks for pointing those out, you're quite right.

[vince-fugnitto]

The changes look good to me 👍

• the added unit-tests tests pass
• updating editor preferences now take effect (ex: whitespace, minimap)
• using language id overrides in preferences also work well (verified with whitespaces)