style: idiomatic cleanups in matcher/formatter/filter test suites

[twcclegg]

Summary

Conservative idiomatic-C# pass over TestPhoneNumberMatcher.cs, TestAsYouTypeFormatter.cs, and TestMetadataFilter.cs. These are near line-by-line ports of the upstream Java test suite, so scope was deliberately narrow: only non-assertion helper code was touched, and no assertion logic, expected values, test data, or test method identity was changed.

• TestPhoneNumberMatcher.cs: == null/!= null → is null/is not null, and a string .Equals() → !=, all in the DoTestNumberMatchesForLeniency/DoTestNumberNonMatchesForLeniency helpers (not inside any Assert.* call).
• TestAsYouTypeFormatter.cs, TestMetadataFilter.cs: reviewed, no safe changes found. TestMetadataFilter.cs has one .Equals(null) call, but it's TestEquals_WhenNull_ReturnsFalse deliberately exercising MetadataFilter.Equals's null-argument behavior — left untouched since that's the point of the test.

Test plan

• dotnet build csharp/PhoneNumbers.slnx — 0 warnings, 0 errors
• dotnet test csharp/PhoneNumbers.Test/PhoneNumbers.Test.csproj — 414/414 passing on net8.0 and net10.0 (same count as before)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.42%. Comparing base (e75ae83) to head (9516f9e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #420   +/-   ##
=======================================
  Coverage   87.42%   87.42%           
=======================================
  Files          41       41           
  Lines        3856     3856           
  Branches      990      990           
=======================================
  Hits         3371     3371           
  Misses        283      283           
  Partials      202      202           

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