Restore existing test comments and keep variadic assertion tests

[selkamand]

Motivation

• Restore test commentary that was accidentally removed while adding the variadic assertion, preserving package test documentation and style.

Description

• Restored existing descriptive comments in tests/testthat/test-assert_functions.R while preserving the new per-expectation comments and blank-line formatting for the assert_function_variadic() block.
• Kept the previously added variadic assertion implementation and artifacts including function_variadic_advanced() in R/assert_functions.R, the exported assert_function_variadic in NAMESPACE, man/assert_function_variadic.Rd, and the NEWS.md entry.
• Committed the updated tests so test descriptions are consistent and readable without changing existing test logic.

Testing

• Ran devtools::document() successfully with updated documentation generated.
• Ran devtools::test() and all tests passed ([ FAIL 0 | WARN 0 | SKIP 0 | PASS 995 ]).
• Ran devtools::check() which completed with 0 errors, 0 warnings and 1 NOTE (unable to verify current time).

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[chatgpt-codex-connector]

Treat primitives with NULL args() as variadic

assert_function_variadic() relies on func_supports_variable_arguments(), which in turn inspects names(formals(args(func))). For several base primitives (e.g., subsetting [/[[), args() returns NULL even though the function accepts ..., so this check returns FALSE and the new assertion incorrectly fails for genuinely variadic primitives. This means callers trying to assert variadic behavior on such primitives will get a false negative. Consider a fallback that treats args(func) == NULL differently (or a different detection strategy) so primitives with undocumented signatures aren’t misclassified.

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