test: add walk() coverage for tuple, set, and nested container types

[Fieldnote-Echo]

Summary

walk() recursively sanitizes nested data structures (dicts, lists, strings). The current 14 tests cover dicts, lists, and strings well, but there are no tests for tuples, sets, or mixed nested containers.

What to do

Add a TestWalkContainerTypes class to tests/test_walk.py:

1. Tuple with hostile string — walk(("hello", "n\u0430vi")) — document whether tuples are traversed or passed through
2. Set with hostile string — walk({"hello", "n\u0430vi"}) — document behavior
3. Deeply nested mixed — walk({"a": [("b", "\u0430")]}) — dict containing list containing tuple
4. Frozen containers — walk({"key": frozenset(["n\u0430vi"])})
5. Original not mutated — verify walk() deep-copies and doesn't modify input for all container types

The goal is to document and test the current behavior, even if some types pass through unsanitized. If you find that tuples/sets should be traversed, note it in the PR for discussion.

Files

• tests/test_walk.py — add new test class

Hints

• Read src/navi_sanitize/_pipeline.py for the walk() implementation
• walk() uses deepcopy — original data is never modified
• Cyrillic \u0430 (а) is a good test char — it maps to Latin a
• Run tests with: uv run pytest tests/test_walk.py -v --benchmark-disable