build(deps): update NuGet packages

[glennawatson]

Summary

Update the NuGet dependencies.

• Use ReactiveUI 26.0.1.
• Update the WPF and WinForms sample dependencies to 26.0.1.

Why

Pick up the XAML binding generator performance fix.

Breaking changes

None.

How this was verified

Built the solution. Ran the existing test suite.

Notes for the reviewer

Only package versions change.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5cd7c40) to head (1401b32).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #991   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines          694       694           
  Branches       104       104           
=========================================
  Hits           694       694           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.