refactor: modularize replace_rarrows and segment-splitting logic

[bhagathkrishnacdac]

Description
This PR refactors replace_rarrows inside sugar.py to resolve cognitive complexity. The original monolithic function, which managed lexical analysis, string slicing, token boundaries, and nested gate checks, has been broken down into single-responsibility helpers.

Key Changes

• Separated Tokenizer Scans (find_arrow_positions): Moved the Python 2/3 lexical analysis scan for right-arrow operators (->) into an isolated helper.
• Streamlined Segment Splitting (split_string_by_arrows): Extracted the complex string index/offset tracking loops used to partition configuration strings. Replaced assert False with an explicit RuntimeError to prevent optimizations from bypassing errors.
• Decoupled Gate Transformation:
  • Extracted process_output_gate and process_input_gate to independently handle colon searches and gate expressions formatting.
  • Extracted parenthesize into a standalone, safe utility function.