SerializingDependencyNodeVisitor holds mutable depth state and is not thread-safe/reentrant

## Summary
`SerializingDependencyNodeVisitor` keeps traversal state in instance fields (`depth`), mutated in `visit(...)`/`endVisit(...)` and read in `getTree(...)`. This makes the visitor neither thread-safe nor safely reentrant:
- Two threads sharing one visitor instance interleave and corrupt the indentation output.
- The same visitor reused for two separate graphs carries stale `depth` between runs.

## Affected code
`src/main/java/org/apache/maven/shared/dependency/graph/traversal/SerializingDependencyNodeVisitor.java`
- `depth` field: line 53
- mutated in `visit(...)`: lines 113-118 and `endVisit(...)`: lines 132-133
- read in `getTree(...)`: lines 89-96

## Impact
`SerializingDependencyNodeVisitor` is the type exposed by `DefaultDependencyNode.getTree(...)`; any caller that reuses or shares the visitor across nodes/threads gets garbled or incorrect output.

## Suggested fix
Make the depth state local to each traversal (e.g. pass a per-run state object through the visitor, or document that each traversal must use a fresh instance), and add a test showing reuse produces correct output.

Read src/main/java/org/apache/maven/shared/dependency/graph/traversal/SerializingDependencyNodeVisitor.java, starting with getTree(...), visit(...), and endVisit(...), to trace the depth state across a traversal. Add a regression test showing that reusing the visitor produces correct output, and verify that shared or repeated traversals no longer corrupt indentation.

Written by the indexing model from the issue text.