Validate missing threshold inputs for length and comparison assertions

[selkamand]

Motivation

• Improve error messages when callers pass NA/NaN as threshold arguments to length/comparison assertions by surfacing clear assertion errors.
• Centralize and harden threshold validation for numeric comparison assertions so comparisons never execute with missing/non-finite thresholds.
• Ensure helper predicates return strict logical scalars so downstream checks behave deterministically.

Description

• Added explicit missing/NaN guards to all assert_length*() variants so length = NA/NaN returns "'length' must not be missing" rather than causing missing value where TRUE/FALSE needed (changes in R/assert_length.R).
• Introduced validate_minimum, validate_maximum, and validate_between_thresholds small validators and wired them into comparison assertion chains (assert_all_* and assert_between/assert_all_between) to validate minimum, maximum, and inclusive before comparisons (changes in R/assert_compare.R).
• Hardened is_whole_number() to return a strict logical scalar and treat missing/non-finite/non-scalar values as FALSE (change in R/is_functions.R).
• Added regression tests covering missing length, minimum, maximum, and updated is_whole_number() expectations (changes in tests/testthat/), and updated NEWS.md to document the user-visible behavior fix.

Testing

• Ran devtools::document() successfully to update docs.
• Ran devtools::test() and all tests passed after fixes (final test run reported all tests passing).
• Ran devtools::check(); package build and checks executed but R CMD check reported one warning about missing system qpdf and one note about time verification, causing devtools::check() to exit with a non-zero status due to the warning; the code changes themselves did not produce R errors during checks.
• Performed targeted runtime spot-checks via devtools::load_all() for assert_length(..., NaN), assert_greater_than(..., NaN), and assert_between(..., maximum = NaN) which returned the new informative assertion messages as expected.

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Codex Task