test: add edge case coverage for path_escaper

[Fieldnote-Echo]

Summary

The path_escaper has 4 tests covering basic traversal attacks, but several edge cases are untested.

What to do

Add test methods to the TestPathEscaper class in tests/test_escapers.py:

1. Multiple consecutive slashes — path_escaper("foo///bar") should normalize
2. Dot-only inputs — path_escaper("..") and path_escaper(".") alone
3. Dots within filenames — path_escaper("file..txt") and path_escaper("..hidden") should be preserved (not path components)
4. Encoded traversal artifacts — path_escaper("foo\x00/../bar") (null byte + traversal)
5. Trailing traversal — path_escaper("foo/bar/../../")

If any test reveals a bug, note it in the PR — fixing it is a bonus but not required.

Files

• tests/test_escapers.py — add to TestPathEscaper

Hints

• Read src/navi_sanitize/escapers/_path.py to understand the regex logic
• Note that path_escaper runs after the main pipeline (null bytes are already stripped by then), but test the escaper in isolation too
• Run tests with: uv run pytest tests/test_escapers.py -v --benchmark-disable