Clear escape delimiter buffer before peek in isEscapeDelimiter

[rootvector2]

isEscapeDelimiter() peeks the next characters into the reused escapeDelimiterBuf look-ahead without clearing it first, so a truncated escaped multi-character delimiter at EOF is completed from the stale bytes of an earlier full escaped delimiter and the lexer appends a delimiter the input never contained. clearing the buffer before the peek mirrors the delimiterBuf reset added in nextToken for the plain multi-character delimiter path.

found while reading the partial-delimiter-at-EOF fix (CSV-324): the escaped-delimiter sibling was left unguarded.

for delimiter [|] and escape !, parsing x![!|!]y![!| returns x[|]y[|] before the change and the correct x[|]y![!| after. regression tests added in LexerTest and CSVParserTest (both fail without the one-line change).

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[garydgregory]

Thank you @rootvector2

PR merged 🚀