DefaultDependencyNode.getChildren() exposes the mutable internal list; class lacks equals/hashCode

## Summary
Two API-hygiene problems on `DefaultDependencyNode`:

1. `getChildren()` returns the internal mutable list (`src/main/java/org/apache/maven/shared/dependency/graph/internal/DefaultDependencyNode.java:136-138`). Callers can add/remove/replace children and corrupt the graph. The builders wrap children in `Collections.unmodifiableList` when constructing (`DefaultDependencyCollectorBuilder.java:233`, `DefaultDependencyGraphBuilder.java:161`), so the mutability is mostly incidental — but the `setChildren`/`getChildren` contract leaves the door open, and `BuildingDependencyNodeVisitor` relies on mutating children of the freshly cloned nodes (so it cannot simply be made immutable everywhere without a builder).

2. The class does not override `equals`/`hashCode`. Combined with identity-based comparisons (see the related `AncestorOrSelfDependencyNodeFilter` issue), value-based matching against rebuilt/cloned trees is impossible.

## Suggested fix
Document the mutation contract clearly, or return an unmodifiable view from `getChildren()`; and add value-based `equals`/`hashCode` (e.g. keyed on artifact coordinates) if identity semantics are not intentional.

Start with DefaultDependencyNode.java at getChildren and setChildren, then inspect the builder call sites in DefaultDependencyCollectorBuilder.java and DefaultDependencyGraphBuilder.java. Read how BuildingDependencyNodeVisitor mutates cloned nodes and review the related AncestorOrSelfDependencyNodeFilter issue before deciding the mutation and equality contracts. Done means the chosen contracts are documented or enforced consistently, with matching hash behavior and tests covering them.

Written by the indexing model from the issue text.