Fix: enforce subtree depth limit for tree cascade children

[Rom1-B]

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !45061

The TreeDropdownChildrenController was stripping the level <= N condition from the WHERE clause before querying child items, causing the subtree depth limit configured on a Tree Cascade Dropdown question to be silently ignored.
Removing the unset restores the restriction so that AJAX child loading respects the configured depth.

Screenshots (if appropriate):

[stonebuzz]

The new test only exercises the children query path (verifying Level3 is excluded once depth is exceeded). It never exercises the parent_is_selectable path, which is fed by the same $condition_param that this fix restores. Can you add a test case where the controller is invoked with parent_id set to an item whose own level already exceeds subtree_depth (e.g. level3), and assert the response no longer renders the selectable placeholder (value="0") that parent_is_selectable controls in the twig template?

[Rom1-B]

done