fix(decorators): guard _get_fn_argnames against empty positional args (closes #2422)

[feiiiiii5]

Root Cause

_get_fn_argnames() in pandera/decorators.py accessed arg_spec_args[0] without checking if the list was empty. Functions with only keyword-only args (def f(*, x)), variadic args (def f(*args)), or keyword-variadic args (def f(**kwargs)) have an empty args list from inspect.getfullargspec, causing IndexError: list index out of range.

Fix

Add an early return when arg_spec_args is empty, before the [0] access:

if not arg_spec_args:
    return arg_spec_args

This is safe because there are no self/cls args to exclude when the positional args list is empty.

Test

• 7 regression tests (tests/pandas/test_get_fn_argnames_empty.py)
  • keyword-only args (exact repro from Bug: _get_fn_argnames raises IndexError for functions with only keyword-only args #2422)
  • variadic args only (*args)
  • kwargs only (**kwargs)
  • no args at all
  • Normal: regular function returns correct argnames
  • Normal: method excludes self
  • Normal: mixed positional + keyword-only returns only positional
• All 7 tests pass
• DCO signed

Diff scope

2 files changed, +90/-0 lines (5 lines fix + 86 lines tests)

AI Disclosure

AI-assisted debug/initial draft/testing. Root cause from issue #2422 analysis. Tests verified locally. Human review of diff scope and correctness.