[rosebyte]

Fixes #7777

Replace all 261 fluent assertion usages across four test projects with built-in xUnit assertions, removing a redundant dependency and making the tests consistent with the rest of the repository.

• Preserve ordered and unordered collection comparisons, floating-point tolerances, numeric thresholds and exact versus derived exception-type expectations.
• Remove all 20 FluentAssertions imports, the four package references, the central package entry and the version property.
• Update assertion examples and repository guidance to use xUnit. Leave historical release notes unchanged.

No production code changes or new assertion helpers.

[codecov]

Codecov Report

❌ Patch coverage is 97.72727% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.80%. Comparing base (ead44cd) to head (abce31b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...icrosoft.ML.AutoML.Tests/TrialResultManagerTest.cs	0.00%	4 Missing ⚠️
...Microsoft.ML.AutoML.Tests/AutoMLExperimentTests.cs	95.65%	1 Missing ⚠️
...est/Microsoft.ML.Fairlearn.Tests/GridSearchTest.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7822      +/-   ##
==========================================
- Coverage   70.82%   70.80%   -0.02%     
==========================================
  Files        1421     1421              
  Lines      280032   280000      -32     
  Branches    27620    27620              
==========================================
- Hits       198329   198262      -67     
- Misses      73543    73578      +35     
  Partials     8160     8160              

Flag	Coverage Δ
Debug	70.80% <97.72%> (-0.02%)	⬇️
production	64.24% <ø> (-0.02%)	⬇️
test	90.77% <97.72%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
test/Microsoft.ML.AutoML.Tests/AutoFitTests.cs	92.87% <100.00%> (ø)
.../Microsoft.ML.AutoML.Tests/ColumnInferenceTests.cs	96.29% <100.00%> (ø)
test/Microsoft.ML.AutoML.Tests/EntityTest.cs	100.00% <100.00%> (ø)
.../Microsoft.ML.AutoML.Tests/GridSearchTunerTests.cs	84.00% <100.00%> (ø)
...Microsoft.ML.AutoML.Tests/MLContextManagerTests.cs	100.00% <100.00%> (ø)
...rosoft.ML.AutoML.Tests/StopTrainingManagerTests.cs	100.00% <100.00%> (ø)
....ML.AutoML.Tests/SweepableEstimatorPipelineTest.cs	95.34% <100.00%> (ø)
...icrosoft.ML.AutoML.Tests/SweepableExtensionTest.cs	96.00% <100.00%> (ø)
...L.AutoML.Tests/TrainValidaionDatasetManagerTest.cs	100.00% <100.00%> (ø)
test/Microsoft.ML.AutoML.Tests/TunerTests.cs	100.00% <100.00%> (ø)
... and 10 more

... and 10 files with indirect coverage changes

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