Reject negative array indices in JSON pointers

[fhgffy]

Resolver.lookup("#/-1") currently resolves to the last array element. Python's negative indexing leaks through Resource.pointer, even though JSON Pointer array indices are unsigned under RFC 6901 section 4.

Reject negative indices with PointerToNowhere. Object members named "-1" still resolve normally. The regression tests cover three negative indices and the object-key case; the three array cases fail on the original code.

This is separate from #387, which handles tokens that cannot be converted to an integer.

Validation on Python 3.14:

• Full suite, including the referencing-suite submodule: 507 passed, 132 xfailed, 406 subtests passed.
• Ruff checks and formatting, Pyright, and sdist/wheel builds passed.