Track opening spans for unclosed delimiters

[leynos]

Summary

• record spans when opening delimiters and surface them in ParseError::UnclosedDelimiter
• expose span-aware open_delimiter helper
• document that unclosed delimiter errors point to the opening token

Testing

• make fmt
• make lint
• make test
• make markdownlint

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https://chatgpt.com/codex/tasks/task_e_689553fd1f5083229549543ef2561574

Summary by Sourcery

Track and report the spans of opening delimiters in the parser to improve UnclosedDelimiter diagnostics and expose a span-aware open_delimiter helper.

New Features:

• Expose a span-aware open_delimiter helper for recording opening token ranges

Enhancements:

• Record opening delimiter spans in DelimStack entries and return them in unclosed error reporting
• Surface the correct opening token span in ParseError::UnclosedDelimiter
• Refactor handle_opening_delimiter to pass token text ranges to the delimiter stack

Documentation:

• Document that unclosed delimiter errors highlight the opening delimiter’s position

Tests:

• Update unclosed_angle_error test to assert the reported span matches the opening token