test(azure-functions): replace NSubstitute with TUnit.Mocks in integration tests

[samtrion]

Summary

• Removes the last NSubstitute usage in the repository — the IInvocationFeatures mock in NetEvolve.Http.Correlation.Azure.Functions.Tests.Integration — and drops the package entirely.
• Bumps TUnit and TUnit.Mocks to 1.63.0. This bump is part of the fix, not an incidental dependency update.

Why it works now

Two separate blockers, both resolved:

1. Compile time (CS1061). SetupHttpRequestFeature took the feature collection as IInvocationFeatures. On the interface type, Get<T>() is the real method returning T, so there is nothing to chain .Returns() onto. Typing the parameter as Mock<IInvocationFeatures> — matching the existing Mock<FunctionContext> parameter — routes the call through the generated mock. Suggested by @thomhurst in [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.
2. Runtime (version floor). The middleware always calls context.GetInvocationResult(), which internally requests the internal Worker type IFunctionBindingsFeature via Get<T>() — a type this assembly cannot name. Verified locally: on 1.62.0 that call returns null and GetRequired<T>() throws InvalidOperationException: No feature is registered with the type Microsoft.Azure.Functions.Worker.Context.Features.IFunctionBindingsFeature. On 1.63.0 the generated mock auto-stubs it and the tests pass. Both conditions are required; either one alone fails.

Both facts are recorded as comments in TestBase.cs so a future version pin or downgrade does not silently resurrect the failure.

This supersedes the closed #738, which worked around the same problem with a hand-written TestInvocationFeatures plus a DispatchProxy fallback. Those two helper types are no longer needed.

Test plan

• dotnet test — 167/167 passing across net8.0/net9.0/net10.0
• Verified the failure mode on 1.62.0 to confirm the version floor is real

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 (6ebc53f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #740   +/-   ##
=======================================
  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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