#58765)

### What problem does this PR solve?

Related PR: #57204

Problem Summary:

This pull request refactors and improves the `PushDownProject` rule in
the Nereids optimizer, mainly focusing on the logic for pushing down
projections through `UNION` operations. It also introduces a
comprehensive unit test to verify the new logic, making the relevant
methods more testable and robust.

**Refactoring and Logic Improvements:**

* Refactored the `pushThroughUnion` logic by extracting it into a new
static method, making it easier to test and use independently. The main
logic now takes explicit arguments instead of relying on the context
object.
* Improved the handling of projections and child outputs when pushing
down through `UNION`, ensuring correct mapping and replacement of slots.
This includes using regulator outputs for children and constant
expressions, and making the slot replacement logic static for better
testability.

**Testing Enhancements:**

* Added a new unit test class `PushDownProjectTest` to rigorously test
the pushdown logic in various scenarios, including unions with and
without children. The tests verify both the structure and the
correctness of the rewritten plans.

**Code Quality Improvements:**

* Added the `@VisibleForTesting` annotation and imported necessary
dependencies to clarify method visibility and intent for testing.
* Replaced some usages of `Collection` with `List` for better type
safety and clarity in projection handling.

These changes make the projection pushdown logic more modular, testable,
and robust, and provide strong test coverage for future maintenance.