fix: convert DateTimeInterface objects to strings in Entity::toRawArray

[gr8man]

Description
Supersedes #10207. Fixes #8302.

Entity::toRawArray() was returning Time objects instead of primitive strings for date fields. This PR fixes it by converting all DateTimeInterface objects to strings.

Unlike the original PR (#10207), this version safely handles native PHP DateTime objects to prevent Fatal Errors (Object of class DateTime could not be converted to string), by falling back to $value->format('Y-m-d H:i:s') if __toString() is not implemented.

Changes:

• Safely stringify DateTimeInterface objects in Entity::toRawArray().
• Added a test case for native DateTime objects.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide