Clear delimiter buffer before each peek in isDelimiter

[rootvector2]

isDelimiter peeks the next characters into the reused delimiterBuf look-ahead buffer without clearing it, so a non-matching peek earlier in the same token leaves stale chars in the trailing positions and a truncated multi-character delimiter at EOF false-matches. With delimiter [|], input x[a][| is split into x[a] and an empty field, because the earlier [a] peek left ] in the buffer and the trailing [| (only two of the three delimiter chars present before EOF) matches against that stale ]. CSV-324 cleared delimiterBuf once per nextToken, but isDelimiter peeks repeatedly within a token, so the reset has to happen before the peek, the same way isEscapeDelimiter already does it; the once-per-token clear is then redundant and dropped.

Found while auditing the multi-character delimiter look-ahead after CSV-324.

• Read the contribution guidelines for this project.
• Read the ASF Generative Tooling Guidance if you use Artificial Intelligence (AI).
• I used AI to create any part of, or all of, this pull request. Which AI tool was used to create this pull request, and to what extent did it contribute?
• Run a successful build using the default Maven goal with mvn; that's mvn on the command line by itself.
• Write unit tests that match behavioral changes, where the tests fail if the changes to the runtime are not applied. This may not always be possible, but it is a best practice.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Each commit in the pull request should have a meaningful subject line and body.

[garydgregory]

Hello @rootvector2

Thank you for your PR.

Would you please add a test that shows why this is an issue only using the public parsing API?

[rootvector2]

Added testPartialMultiCharacterDelimiterAtEOFAfterMismatch in CSVParserTest that drives this through the public parsing API. Parsing x[a][| with delimiter [|]: without the fix it splits into x[a] and an empty field, with the fix it stays a single field x[a][|. The test fails on master and passes with the patch.

[garydgregory]

@rootvector2

Thank you for your update.

In the future, please make magic strings like "x[a][|" a local variable such that you never have to check that the assertion below is checking for a possibly different value.

[garydgregory]

@rootvector2

Thank you for your update.

In the future, please make magic strings like "x[a][|" a local variable such that you never have to check that the assertion below is checking for a possibly different value.

[garydgregory]

@rootvector2
Please see my comments for future PRs. Merged and thank you! 🚀

[rootvector2]

makes sense, will pull magic strings like that into locals going forward. thanks for the merge.