fix: exclude trailing whitespace from quoted hex literals

[minleejae]

The lexer includes trailing spaces in quoted hex tokens. For SELECT X'6162' AS v, HexValue.getDigits() consequently includes a quote, getLong() throws, and each parse/render cycle adds whitespace. JSON_VALUE DEFAULT expressions and generated columns have the same problem.

Factor the quoted segment into a private lexer fragment and allow separating spaces only when followed by another quoted segment. S_HEX now ends at its final quote. Existing hex spellings and legacy segmented forms remain accepted; the public HexValue API is unchanged.

Validation: ./gradlew check passed. Tests cover numeric/string/blob accessors, whitespace before aliases and FROM, repeated round trips, existing token forms, and editing a JSON_VALUE default through the AST. Five reproductions were executed on MySQL 8.4.11 before and after rendering, with identical results. This fixes token boundaries and AST stability; the original SQL was already executable in MySQL.

Combined validation with #2682, #2683, #2684, #2685 and #2686: all five changes merge without conflicts; the integrated ./gradlew check passes (8,190 tests, 25 skipped). All 254 MySQL 8.4.11 fixtures parse, render and reparse, and execute with results identical to the original SQL.