Add p.until_max() for length-limited string parsing in XML-style formats
Implements length-limited `until` parser that excludes delimiters while
constraining maximum character count. Generates O(max_length) grammar
rules with proper delimiter exclusion trie.
Updates Seed-OSS parser to use until_max for string parameters with
maxLength schema constraint, enabling required parameter enforcement.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>