.Net: Add Filter to TextSearchProvider (#12662)

### Motivation and Context

1.Enable the Agent mode to utilize underlying Filter for filtering

### Description

Same as Top, directly exposing the Filter of TextSearchOptions

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

---------

Co-authored-by: westey <[email protected]>

Author: qideqian

dotnet/src/SemanticKernel.Core/Data/TextSearchBehavior/TextSearchProvider.cs

@@ -75,7 +75,7 @@ public override async Task<AIContext> ModelInvokingAsync(ICollection<ChatMessage
 
         var searchResults = await this._textSearch.GetTextSearchResultsAsync(
             input,
-            new() { Top = this.Options.Top },
+            new() { Top = this.Options.Top, Filter = this.Options.Filter },
             cancellationToken: cancellationToken).ConfigureAwait(false);
 
         var results = await searchResults.Results.ToListAsync(cancellationToken).ConfigureAwait(false);
@@ -96,7 +96,7 @@ internal async Task<string> SearchAsync(string userQuestion, CancellationToken c
     {
         var searchResults = await this._textSearch.GetTextSearchResultsAsync(
             userQuestion,
-            new() { Top = this.Options.Top },
+            new() { Top = this.Options.Top, Filter = this.Options.Filter },
             cancellationToken: cancellationToken).ConfigureAwait(false);
 
         var results = await searchResults.Results.ToListAsync(cancellationToken).ConfigureAwait(false);

dotnet/src/SemanticKernel.Core/Data/TextSearchBehavior/TextSearchProviderOptions.cs

@@ -33,6 +33,11 @@ public int Top
         }
     }
 
+    /// <summary>
+    /// Gets or sets the filter expression to apply to the search query.
+    /// </summary>
+    public TextSearchFilter? Filter { get; init; }
+
     /// <summary>
     /// Gets or sets the time at which the text search is performed.
     /// </summary>

dotnet/src/SemanticKernel.UnitTests/Data/TextSearchProviderTests.cs

@@ -263,4 +263,56 @@ public async Task ModelInvokingShouldUseOverrideContextFormatterIfProvidedAsync(
         // Assert
         Assert.Equal("Custom formatted context with 2 results.", result.Instructions);
     }
+
+    [Fact]
+    public async Task SearchAsyncRespectsFilterOption()
+    {
+        // Arrange
+        var mockTextSearch = new Mock<ITextSearch>();
+        var searchResults = new Mock<IAsyncEnumerable<TextSearchResult>>();
+        var mockEnumerator = new Mock<IAsyncEnumerator<TextSearchResult>>();
+
+        // Simulate the filtered results
+        var filteredResult = new TextSearchResult("Filtered Content") { Name = "FilteredDoc", Link = "http://example.com/filtered" };
+        var results = new List<TextSearchResult> { filteredResult };
+
+        mockEnumerator.SetupSequence(e => e.MoveNextAsync())
+            .ReturnsAsync(true)
+            .ReturnsAsync(false);
+
+        mockEnumerator.SetupSequence(e => e.Current)
+            .Returns(filteredResult);
+
+        searchResults.Setup(r => r.GetAsyncEnumerator(It.IsAny<CancellationToken>()))
+            .Returns(mockEnumerator.Object);
+
+        TextSearchFilter? capturedFilter = null;
+        mockTextSearch.Setup(ts => ts.GetTextSearchResultsAsync(
+                It.IsAny<string>(),
+                It.IsAny<TextSearchOptions>(),
+                It.IsAny<CancellationToken>()))
+            .Callback<string, TextSearchOptions?, CancellationToken>((q, opts, ct) =>
+            {
+                capturedFilter = opts?.Filter;
+            })
+            .ReturnsAsync(new KernelSearchResults<TextSearchResult>(searchResults.Object));
+
+        var filter = new TextSearchFilter().Equality("Name", "FilteredDoc");
+        var options = new TextSearchProviderOptions
+        {
+            Filter = filter
+        };
+
+        var provider = new TextSearchProvider(mockTextSearch.Object, options: options);
+
+        // Act
+        var result = await provider.SearchAsync("Sample user question?", CancellationToken.None);
+
+        // Assert
+        Assert.Contains("Filtered Content", result);
+        Assert.Contains("SourceDocName: FilteredDoc", result);
+        Assert.Contains("SourceDocLink: http://example.com/filtered", result);
+        Assert.NotNull(capturedFilter);
+        Assert.Equal(filter, capturedFilter);
+    }
 }