Fix IndexOutOfRangeException when a comment line ends on the buffer boundary

[scepion1d]

Why?

CsvParser throws System.IndexOutOfRangeException when parsing a comment (or blank) line whose trailing line ending lands exactly on the read-buffer boundary. It reproduces reliably at comment-line lengths of the form k × BufferSize − 2 for power-of-two k (2, 4, 8, …), because FillBuffer doubles the buffer when a record outgrows it. Any input with a sufficiently long comment line can hit this

What?

• Edit: src/CsvHelper/CsvParser.cs - In ReadLine, a comment/blank line is consumed by ReadBlankLine, which reads the line including its \r\n. On the state-machine resume path (after a buffer refill), completion fell through to c = buffer[bufferPosition] without re-checking bufferPosition < charsRead. When the line ending aligned with the buffer boundary (bufferPosition == charsRead == buffer.Length), this read past the end of the buffer. The fix loops back (continue) when a BlankLine state completes, so the bound is re-evaluated - mirroring the already-safe fresh-detection path.
• Tests: parameterized by line ending (CRLF and LF) - an exhaustive comment-length sweep (sync + async) across buffer sizes 16/32/64, plus explicit  k × BufferSize  boundary cases at 4096/8192. The CRLF cases reproduce the crash without the fix; the LF cases lock in that LF stays safe.