Bring the 4.0.0 CHANGELOG entry up to date before release

Problem/Motivation

The 4.0.0 entry in CHANGELOG.txt was written partway through the Drupal 12 work and has not been updated since. It is dated 2026-06-27 — when the D12 branch was drafted, not when the release shipped — and it is missing six commits' worth of changes that landed on 4.0.x afterwards.

Missing from the entry:

• 9cd6a4c — the image component settings were never read (issue #3614068). config/install shipped layout_builder_kit.settings with flat keys while the settings form and both image-bearing components read layout_builder_kit.image_component with nested keys, so a fresh install had no upload folder and no file-extension validation. Includes layout_builder_kit_update_10001() to migrate administrator-saved values.
• 2280f63 — missing : void return type on submitConfigurationForm() (issue #3531242).
• 7fc2c10 — private service properties widened to protected. DependencySerializationTrait::__sleep() calls get_object_vars() from the trait's scope, so private properties declared on a subclass are invisible to it and get serialized raw — the same failure mode as the "database connection is not serializable" reports. Also renamed the functional test class, which had never been collected by PHPUnit because its name ended in "Tests".
• 8ea0b83, dde87af, 75c7a60 — CI now builds against Drupal 12. Until these landed, every pipeline on this Drupal-12-only branch was silently testing against Drupal 11, because the config pinned no core version and the templates default to current stable.

A changelog is the only place a site builder finds out what changed. Shipping 4.0.0 with an entry that omits a security-relevant configuration fix and misrepresents what CI verified would be misleading.

Steps to reproduce

Compare the entry against the branch:

git log --oneline 6a81a15..origin/4.0.x

Six commits, none of them reflected in CHANGELOG.txt.

Proposed resolution

Rewrite the 4.0.0 entry so it accurately describes the release, and set its date to the day the release is actually tagged. Keep the existing "Note on HTML output differences when upgrading a site to Drupal 12" section — it is still correct and is the most useful part of the entry for anyone planning an upgrade.

Remaining tasks

• Update the entry (merge request attached).
• Do not close this issue until 4.0.0 is actually tagged — the entry describes a release that does not exist yet, and the date should be corrected to the real release date at that point.