Enhanced OpenXmlImporter with new functionality for retrieving comments and notes

[michelebastione]

New comment retrieval API

This PR adds support for retrieving both threaded comments and legacy notes from Excel worksheets through the new RetrieveComments and RetrieveCommentsAsync methods in the OpenXmlImporter class, and thus closes #440.

Extracted comment data includes:

• Cell address
• Author information (name and provider id)
• Creation timestamp
• Comment text
• Nested replies with their own metadata

Extracted note data includes:

• Cell address
• Author name (if available)
• Note text

[gemini-code-assist]

Code Review

This pull request implements functionality to retrieve threaded comments and legacy notes from Excel files, adding new models and updating the OpenXmlReader and OpenXmlImporter. It also enhances stream management for modern .NET versions and standardizes namespace constants. Review feedback suggests several improvements for robustness and performance, including safer null handling for paths, culture-invariant date parsing, more efficient file entry lookups, and better handling of legacy comment attributes and text nodes.