Preserve large integer strings in intcomma

[x0Lazarus]

intcomma(str(10**400 + 123)) currently returns "+Inf", although the same finite value passed as an integer is formatted correctly. The string path checks float(value) before attempting integer parsing, so values beyond the floating-point range are mistaken for infinity.

With the default precision, parse integer strings with int() first, after normalizing the locale's separators. Inputs that are not integer strings continue through the existing float/non-finite handling. This extends the default-precision behavior fixed for integer inputs in #392 to plain and already-grouped integer strings.

The regression tests cover both signs, preserve the final 123, and check German and French grouping. Compatibility cases retain the existing handling of decimal strings, exponent strings, invalid input, and non-finite values. The existing float check is retained when ndigits is explicitly set, with controls for both signs beyond the float range.

Validation on Windows with Python 3.12:

• Four plain/grouped string regressions fail on the original source and pass with the change.
• Focused number and localization tests: 49 passed.
• Full suite with doctests and benchmark timing disabled: 799 passed, 69 skipped. German, French and Portuguese catalogs were compiled; remaining translation cases and gettext tool checks were skipped.
• Ruff and Black pass for the three changed files; mypy passes for all 14 source/test files; git diff --check passes.