grammar : parse \- in char classes as literal hyphen (#27591)

* grammar : accept "\-" escape in character classes

gbnf_escape_char_class() escapes '-' as "\-" but parse_char() rejected
that escape, so generated tool-call grammars failed to parse.

Assisted-by: Claude Code <claude@anthropic.com>

* tests : add parser test for "\-" in char classes

Assisted-by: Claude Code <claude@anthropic.com>

* tests : add integration test for "\-" in char classes

Assisted-by: Claude Code <claude@anthropic.com>

* tests : drop integration and parser tests