Reject NaN in scalar sign assertions

[dhairyajangir]

NaN currently passes assert_scalar_positive, assert_scalar_non_negative, and assert_scalar_negative: each helper rejects the opposite relation, and every ordered comparison with NaN is false. This lets an invalid scalar satisfy mutually exclusive sign assertions.

Check each required relation directly and negate its result, consistent with assert_scalar_in. Existing error messages, scalar type checks, signed-zero behavior and permitted infinities remain unchanged.

Validation on Linux, Python 3.12.14, JAX/JAXlib 0.11.2:

• The three new NaN regression cases fail on the original implementation with AssertionError not raised and pass with the fix.
• Full chex/_src/asserts_test.py: 313 passed, 48 skipped, 85 subtests passed. The baseline has the same 48 skips.
• Assertion-module coverage: 90% with branch measurement enabled.
• Eighteen manual boundary controls for signed zeros, infinities and booleans passed.
• Repository Flake8 error/spacing checks on both changed files and source/wheel builds passed.
• Targeted Pylint using the repository configuration reports the same existing refactor diagnostics as the base; no new diagnostics.

The complete multi-version test.sh workflow and documentation build were not run locally.

Developed, tested, and independently reviewed with Codex assistance.

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