Docstring extraction should handle prefixed strings (r/f/u/b)

[bbopen]

Summary

extractDocstring only strips raw triple quotes without handling string prefixes. Docstrings like r"""...""" or f"""...""" are returned with prefixes/quotes intact, polluting generated docs.

Evidence

• src/core/analyzer.ts extractDocstring only checks for """/'''/"/'.

Proposed fix

• Normalize optional string prefixes (r, u, f, b, and combinations like rf) before stripping quotes.
• Add tests for prefixed docstrings.

Acceptance criteria

• Prefixed docstrings are stripped cleanly and returned without prefixes or outer quotes.