chore(tests): replace NSubstitute with a DispatchProxy-based workaround

[samtrion]

Summary

• Removes the NSubstitute dependency from NetEvolve.Http.Correlation.Azure.Functions.Tests.Integration.
• Bumps TUnit and TUnit.Mocks to 1.63.0.
• This is a workaround, not a full migration to TUnit.Mocks. TUnit.Mocks is a compile-time source generator, so it cannot configure a mock's generic method for a type argument this assembly can't name — in this case the internal Azure Functions Worker type IFunctionBindingsFeature, which the SDK requests internally via FunctionContext.GetInvocationResult(). See [TUnit.Mocks] Get<T>() can't be configured for T inaccessible to the calling assembly (unlike NSubstitute's runtime-proxy auto-recursion) thomhurst/TUnit#6514 for the upstream discussion; confirmed against 1.63.0 that the gap still exists.
• The workaround: TestInvocationFeatures, a small hand-written IInvocationFeatures, falls back to DispatchProxy.Create<T, NullReturningProxy>() for any unregistered interface T. Since DispatchProxy.Create is invoked with T as a generic parameter resolved at the call site via reflection, it works even for types this assembly has no visibility into.

Test plan

• dotnet test — 167/167 passing, including NetEvolve.Http.Correlation.Azure.Functions.Tests.Integration on net8.0/net9.0/net10.0

Refs #723, thomhurst/TUnit#6514

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.71%. Comparing base (1e56c99) to head (5756406).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #738   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files          20       20           
  Lines         175      175           
  Branches       22       22           
=======================================
  Hits          171      171           
  Misses          2        2           
  Partials        2        2           

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[samtrion]

Found the clean solution