Fix broken CI

[martincostello]

Remove dotnet-validate from .NET tools manifest to avoid build failure on macOS.

Works around dotnet/sdk#46857.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.35%. Comparing base (4e6f93f) to head (5d28411).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2497   +/-   ##
=======================================
  Coverage   85.35%   85.35%           
=======================================
  Files         312      312           
  Lines        7465     7465           
  Branches     1121     1121           
=======================================
  Hits         6372     6372           
  Misses        908      908           
  Partials      185      185           

Flag	Coverage Δ
linux	85.35% <ø> (ø)
macos	85.35% <ø> (ø)
windows	85.32% <ø> (ø)

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