[huuan26]

Fixes #16122

Summary

When an argument to -f, -s, or -w\ overflowed \usize, \opt_parsed\ unconditionally converted \IntErrorKind::PosOverflow\ into \usize::MAX\ without checking whether the remaining characters in the string were non-digit garbage (e.g. \18446744073709551616x). This caused values with invalid suffixes to be silently accepted.

This PR adds a check \�rg_str.chars().all(|c| c.is_ascii_digit())\ on \IntErrorKind::PosOverflow\ so that:

• Valid oversized numeric counts (e.g. \18446744073709551616) remain accepted and clamped to \usize::MAX.
• Malformed inputs with trailing garbage (e.g. \18446744073709551616x) are properly rejected with an invalid argument error, matching GNU \uniq.

Verification

Added test cases in \ ests/by-util/test_uniq.rs\ verifying:

• -f, -s, and -w\ fail with code 1 when supplied with \18446744073709551616x.
• Purely numeric overflowing values remain accepted.
  All tests pass.