Preserve row delimiters sharing a column separator prefix

[OskarEichler]

Summary

When scanning an unquoted field with a multi-character column separator, check for a complete row delimiter before consuming an unmatched column-prefix character. Reuse the existing chunk-aware separator scanners.

Reproduction

CSV.parse("x||y||z|~|", col_sep: "||", row_sep: "|~|", liberal_parsing: true) raises MalformedCSVError: the final row delimiter's first | is consumed as field data.

Verification

54 focused cases pass in both parser modes across three encodings, three diverging delimiter-prefix pairs and varying row counts. The release fails the direct reproduction. Together with the separate writer quote fix, all 1,080 corpus checks pass in each parser mode; neither patch alone addresses the other cause.

This isolated patch passes the unchanged upstream suite in both normal and CSV_PARSER_SCANNER_TEST=yes modes: 527 tests / 4,050 assertions in each mode, zero failures/errors. Commands use Ruby 4.0.6 through rbenv (bundle exec ruby run-test.rb). Syntax and git diff --check pass. The combined release-based consumer branch also passes gem build and rake warning:error rdoc.

No tests were added or changed, per the requesting project's explicit no-new-tests policy. Focused checks and measurements live outside this repository. The upstream no-RuboCop policy is respected. Verification was local on macOS/Ruby 4.0.6; the supported OS/Ruby matrix was not run locally. Current main and installed 3.3.6 share base commit 0873ab362d996f12796c3c3e8998b5be657a9b12; no unrelated changes are included.

Breaking changes and limitations

None intended. This fixes diverging separators that share an initial prefix. Identical delimiters or a full delimiter being a prefix of the other are not claimed unambiguous or newly supported.