related PR #58964

Problem Summary:

This pull request refactors how child physical plans are accessed in the
`ChildrenPropertiesRegulator` class, simplifying the code and improving
test clarity. The main change is the removal of the
`getChildPhysicalPlan` helper method, replacing its usage with direct
access to the plan from the `children` list. The tests are also updated
to build child mocks more locally, improving test isolation and
readability.

Refactoring and Simplification:

* Removed the `getChildPhysicalPlan` method from
`ChildrenPropertiesRegulator`, and replaced its usage in
`visitPhysicalFilter` and `visitPhysicalProject` with direct access to
the child plan via `children.get(0).getPlan()`. This simplifies the code
by eliminating unnecessary indirection.