Fix nullable boolean handling in LINQ filters and add IsFamilyFriendly test

- ProcessEqualityExpression now handles Convert expressions for nullable types (bool?)
- Added explicit boolean-to-lowercase string conversion for Bing API compatibility
- Added test for IsFamilyFriendly == true filter (validates safeSearch query parameter)
- Achieves 100% coverage of all 10 LINQ filter operations
- Note: CA1308 warning acceptable - Bing API requires lowercase boolean strings

Author: alzarei

dotnet/src/Plugins/Plugins.UnitTests/Web/Bing/BingTextSearchTests.cs

@@ -462,6 +462,30 @@ public async Task GenericSearchAsyncWithNoFilterReturnsResultsSuccessfullyAsync(
         Assert.Equal(10, resultList.Count);
     }
 
+    [Fact]
+    public async Task GenericSearchAsyncWithIsFamilyFriendlyFilterProducesCorrectBingQueryAsync()
+    {
+        // Arrange
+        this._messageHandlerStub.AddJsonResponse(File.ReadAllText(WhatIsTheSKResponseJson));
+        ITextSearch<BingWebPage> textSearch = new BingTextSearch(apiKey: "ApiKey", options: new() { HttpClient = this._httpClient });
+
+        // Act
+        var searchOptions = new TextSearchOptions<BingWebPage>
+        {
+            Top = 4,
+            Skip = 0,
+            Filter = page => page.IsFamilyFriendly == true
+        };
+        KernelSearchResults<string> result = await textSearch.SearchAsync("What is the Semantic Kernel?", searchOptions);
+
+        // Assert - Verify LINQ IsFamilyFriendly equality converted to Bing's safeSearch query parameter
+        var requestUris = this._messageHandlerStub.RequestUris;
+        Assert.Single(requestUris);
+        Assert.NotNull(requestUris[0]);
+        // safeSearch is a query parameter, not an advanced search operator
+        Assert.Contains("safeSearch=true", requestUris[0]!.AbsoluteUri);
+    }
+
     #endregion
 
     /// <inheritdoc/>

dotnet/src/Plugins/Plugins.Web/Bing/BingTextSearch.cs

@@ -204,24 +204,41 @@ private static void ProcessExpression(Expression expression, TextSearchFilter fi
     /// <param name="isNegated">True if this is an inequality (!=) expression.</param>
     private static void ProcessEqualityExpression(BinaryExpression binaryExpr, TextSearchFilter filter, bool isNegated)
     {
-        if (binaryExpr.Left is MemberExpression memberExpr && binaryExpr.Right is ConstantExpression constExpr)
+        // Handle nullable properties with conversions (e.g., bool? == bool becomes Convert(property) == value)
+        MemberExpression? memberExpr = binaryExpr.Left as MemberExpression;
+        if (memberExpr == null && binaryExpr.Left is UnaryExpression unaryExpr && unaryExpr.NodeType == ExpressionType.Convert)
+        {
+            memberExpr = unaryExpr.Operand as MemberExpression;
+        }
+
+        // Handle conversions on the right side too
+        ConstantExpression? constExpr = binaryExpr.Right as ConstantExpression;
+        if (constExpr == null && binaryExpr.Right is UnaryExpression rightUnaryExpr && rightUnaryExpr.NodeType == ExpressionType.Convert)
+        {
+            constExpr = rightUnaryExpr.Operand as ConstantExpression;
+        }
+
+        if (memberExpr != null && constExpr != null)
         {
             string propertyName = memberExpr.Member.Name;
             object? value = constExpr.Value;
 
             string? bingFilterName = MapPropertyToBingFilter(propertyName);
             if (bingFilterName != null && value != null)
             {
+                // Convert boolean values to lowercase strings for Bing API compatibility
+                string stringValue = value is bool boolValue ? boolValue.ToString().ToLowerInvariant() : value.ToString() ?? string.Empty;
+
                 if (isNegated)
                 {
                     // For inequality, wrap the value with a negation marker
                     // This will be processed in BuildQuery to prepend '-' to the advanced search operator
                     // Example: language:en becomes -language:en (excludes pages in English)
-                    filter.Equality(bingFilterName, $"-{value}");
+                    filter.Equality(bingFilterName, $"-{stringValue}");
                 }
                 else
                 {
-                    filter.Equality(bingFilterName, value);
+                    filter.Equality(bingFilterName, stringValue);
                 }
             }
             else if (value == null)