feat: add create_work_item and update_work_item MCP tools

[viamus]

Add two new write operations for Azure DevOps work items:

• create_work_item: creates work items with all standard fields, parent linking, tags, and custom fields via additionalFields JSON
• update_work_item: partial updates with null-means-no-change semantics

Includes service layer implementation using JsonPatchDocument, 15 unit tests, and updated documentation.

Description

Brief description of the changes in this PR.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #(issue number)

Changes Made

Testing

• I have tested these changes locally
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have updated the documentation accordingly

Screenshots (if applicable)

Additional Notes