fix: Ensure unmatched optional capture group is treated as null in parseRowColumnSelection

[zigzagdev]

Summary

What I have done

• Added PREG_UNMATCHED_AS_NULL flag to the preg_match call in parseRowColumnSelection()
• Without this flag, PHP sets unmatched named capturing groups to '' (empty string) rather than leaving them absent, causing $found['end'] ?? null to return ''instead of null
• This bug caused single-number selections (e.g. row=1, col=2) to always fail validation when combined with semicolon-separated expressions (e.g. row=1;3, col=1;3)

Where I have changed

• src/FragmentFinder.php: Add PREG_UNMATCHED_AS_NULL to preg_match and simplify $end assignment from $found['end'] ?? null to $found['end']
• src/TabularDataReaderTestCase.php: Add test cases covering following cases
  • Multiple single-number row/column selections (row=1;3, col=1;3)
  • Mixed single and range selections (row=1;3-5)
  • All-invalid single selections (row=0;0)
  • Mixed valid/invalid selections — matchingFirst returns partial results, matchingFirstOrFail throws FragmentNotFound

Test Plan

Expression	What is verified
row=1;3	Two single-number row selections both resolve correctly without a range endpoint
row=1;3-5	A single-number selection and a range selection can coexist in one expression
col=1;3	Two single-number column selections both resolve correctly without a range endpoint
row=0;0	All-invalid single selections yield null from matchingFirst and throw from matchingFirstOrFail
row=0;3 (×2)	When valid and invalid selections are mixed, matchingFirst returns only the valid result while matchingFirstOrFail throws FragmentNotFound

Test Cases Result