bugfix: improve CollectionShouldHaveCountLessThan_CountShouldBeLessThan detection (#398)

Author: Meir017

src/FluentAssertions.Analyzers.Tests/Tips/CollectionTests.cs

@@ -516,6 +516,12 @@ public void CollectionShouldHaveCount_LengthShouldBe_TestNoAnalyzer(string asser
         [Implemented]
         public void CollectionShouldHaveCountLessThan_TestAnalyzer(string assertion) => VerifyCSharpDiagnosticCodeBlock(assertion, DiagnosticMetadata.CollectionShouldHaveCountLessThan_CountShouldBeLessThan);
 
+        [DataTestMethod]
+        [AssertionDiagnostic("(actual.Count() + 1).Should().BeLessThan(k{0});")]
+        [AssertionDiagnostic("(actual.Count() + 1).Should().BeLessThan(6{0});")]
+        [Implemented]
+        public void CollectionShouldHaveCountLessThan_TestNoAnalyzer(string assertion) => DiagnosticVerifier.VerifyCSharpDiagnosticUsingAllAnalyzers(GenerateCode.GenericIListCodeBlockAssertion(assertion));
+
         [DataTestMethod]
         [AssertionCodeFix(
             oldAssertion: "actual.Count().Should().BeLessThan(k{0});",

src/FluentAssertions.Analyzers/Tips/FluentAssertionsAnalyzer.cs

@@ -504,7 +504,7 @@ private static void AnalyzeInvocation(OperationAnalysisContext context, FluentAs
                 return;
             case "BeLessThan" when assertion.IsContainedInType(metadata.NumericAssertionsOfT2):
                 {
-                    if (invocation.TryGetFirstDescendent<IInvocationOperation>(out var invocationBeforeShould))
+                    if (invocation.TryGetSingleArgumentAs<IInvocationOperation>(out var invocationBeforeShould))
                     {
                         switch (invocationBeforeShould.TargetMethod.Name)
                         {