csplit: create final empty file with --suppress-matched to match GNU (fixes #7286)

Problem

• With , uutils csplit failed to create the final empty output file when the last split point consumed the trailing input.
• GNU csplit always creates a final segment after processing patterns; with , that empty final segment is elided.
• This broke the GNU test (tests/csplit/csplit-suppress-matched.pl) and the scenario:

Root cause

• Final-file creation was conditional on there being remaining input after pattern processing. If none remained, no final file was created, contrary to GNU semantics.

Fix (minimal, targeted)

• In , after :
  • If there is remaining input, always create a final split and copy the remainder, then finish.
  • Else, if all patterns were integer-based and is set, create a final (possibly empty) split and finish; elides it when is set.

Tests (Rust integration)

• Added two Rust tests under to lock down GNU-compatible behavior:
  • (expects sizes 2,2,2,0 and a final empty )
  • (final empty file is correctly elided)

Verification

• All tests pass locally. The originally reported case now matches GNU.

Relation to PR #7806

• csplit: rewrite with one function per pattern type #7806 proposes a broader refactor to fix multiple issues, but remains a draft and notes remaining GNU suppress-matched differences.
• This PR provides a small, reviewable fix specifically for csplit: --suppress-matched incorrectly elides last empty file #7286, plus precise integration tests to safeguard behavior.

Fixes #7286