.Net: feat: Modernize samples and documentation for ITextSearch<TRecord> interface (microsoft#10456)

[alzarei]

Modernize samples and documentation for ITextSearch interface

Problem Statement

The existing GettingStartedWithTextSearch samples only demonstrate the legacy ITextSearch interface with clause-based filtering. With the introduction of generic ITextSearch interfaces, developers need clear examples showing both legacy patterns and modern LINQ-based patterns for migration guidance.

Technical Approach

This PR completes the Issue #10456 implementation by updating the GettingStartedWithTextSearch samples to demonstrate the new generic ITextSearch interface with LINQ filtering capabilities. The implementation focuses on developer education and smooth migration paths.

Implementation Details

Sample Structure

• Existing legacy examples preserved unchanged for backward compatibility
• New SearchWithLinqFilteringAsync() method demonstrates modern patterns
• Educational examples showing intended usage patterns for BingWebPage, GoogleWebPage, and VectorStore connectors
• Clear documentation explaining when to use which approach

Educational Content

• Console output provides educational context about modernization benefits
• Code comments explain the technical advantages of type-safe filtering
• Examples show both simple and complex LINQ filtering scenarios
• Clear messaging about availability and migration timeline

Code Examples

Legacy Interface (Preserved)

var legacyOptions = new TextSearchOptions
{
    Filter = new TextSearchFilter()
        .Equality("category", "AI")
        .Equality("language", "en")
};

Modern Interface Examples

// BingWebPage filtering
var modernBingOptions = new TextSearchOptions<BingWebPage>
{
    Filter = page => page.Name.Contains("AI") && page.Snippet.Contains("semantic")
};

// GoogleWebPage filtering
var modernGoogleOptions = new TextSearchOptions<GoogleWebPage>
{
    Filter = page => page.Name.Contains("machine learning") && page.Url.Contains("microsoft")
};

// VectorStore filtering
var modernVectorOptions = new TextSearchOptions<DataModel>
{
    Filter = record => record.Tag == "Technology" && record.Title.Contains("AI")
};

Implementation Benefits

For Developers Learning Text Search

• Clear examples of both legacy and modern interface patterns
• Educational console output explaining the benefits of each approach
• Practical examples showing how to migrate existing filtering code
• Understanding of compile-time safety vs. runtime string validation

For Existing Users

• No disruption to existing code - all legacy examples still work unchanged
• Clear migration path when ready to adopt modern interfaces
• Understanding of when and how to use the new generic interfaces
• Future-proof examples that work today and integrate seamlessly later

For the Semantic Kernel Ecosystem

• Complete sample coverage for the modernized text search functionality
• Educational content supporting developer adoption of new patterns
• Foundation for demonstrating connector-specific implementations

Validation Results

Build Verification

• Command: dotnet build --configuration Release --interactive
• Result: Build succeeded
• Status: ✅ PASSED (0 errors, 0 warnings)

Test Results
Full Test Suite (with external dependencies):

• Passed: 7,042 (core functionality tests)
• Failed: 2,934 (external API configuration issues)
• Skipped: 389
• Duration: 4 minutes 57 seconds

Core Unit Tests (framework only):

• Command: dotnet test src\SemanticKernel.UnitTests\SemanticKernel.UnitTests.csproj --configuration Release
• Total: 1,574 tests
• Passed: 1,574 (100% core framework functionality)
• Failed: 0
• Duration: 1.5 seconds

Test Failure Analysis
The 2,934 test failures are infrastructure/configuration issues, not code defects:

• Azure OpenAI Configuration: Missing API keys for external service integration tests
• Docker Dependencies: Vector database containers not available in development environment
• External Dependencies: Integration tests requiring live API services (Bing, Google, etc.)

These failures are expected in development environments without external API configurations.

Code Quality

• Formatting: dotnet format SK-dotnet.slnx - no changes required (already compliant)
• Code meets all formatting standards
• Documentation follows XML documentation conventions
• Sample structure follows established patterns in GettingStartedWithTextSearch

Files Modified

dotnet/samples/GettingStartedWithTextSearch/Step1_Web_Search.cs (MODIFIED)

Breaking Changes

None. All existing samples continue to work unchanged with new content being additive only.

Multi-PR Context

This is PR 6 of 6 in the structured implementation approach for Issue #10456. This PR provides educational content and sample modernization to complete the comprehensive text search interface modernization, enabling developers to understand and adopt the new LINQ-based filtering patterns.