Multi line text update in work item discussion

**Is your feature request related to a problem? Please describe.**
Yes. Currently, the az boards work-item update --discussion command does not support multi-line comments. When attempting to pass a string with line breaks (either via here-strings, escaped \n, or file input using @file.txt), only the first line is recorded in the work item discussion. This limitation significantly reduces the usefulness of the command for real-world collaboration, where formatted updates or daily activity summaries often require multiple lines for clarity.

**Describe the solution you'd like**
I would like az boards work-item update --discussion to support full multi-line text input, respecting newline characters and preserving formatting when:

Passing multi-line strings via PowerShell or Bash (with \n)

Using here-strings in PowerShell (@"..."@)

Using @file.txt syntax to load the comment from a file

This enhancement would allow users to provide richer, more structured updates directly from the CLI, aligning with the way discussions are typically written in the Azure DevOps portal UI.

**Additional context**
This limitation impacts:
- Automation scripts for daily standups or deployment notes
- DevOps teams using CLI for faster workflow integration
- Users preferring structured bullet points or markdown-style formatting

Workarounds like flattening updates into a single line using | or switching to the REST API are not ideal. Full support for multi-line input in --discussion would improve the CLI’s usability and consistency with the Azure DevOps web interface.