Enforce IHttpClientFactory as required parameter in GetDefaultHttpClient

[Copilot]

Addresses feedback from #148 to eliminate the optional fallback path that instantiates HttpClient directly, which can cause socket exhaustion.

Changes

• RuntimeUtility.GetDefaultHttpClient: Made IHttpClientFactory parameter required (non-nullable), removed ?? new HttpClient() fallback
• Documentation: Updated XML comments to reflect required parameter

Breaking Change

// Before: Optional factory with fallback
public static HttpClient GetDefaultHttpClient(
    IHttpClientFactory? httpClientFactory = null, 
    IUserAgentConfiguration? userAgentConfiguration = null, 
    int timeoutSeconds = 30)
{
    var httpClient = httpClientFactory?.CreateClient() ?? new HttpClient();
    // ...
}

// After: Factory required
public static HttpClient GetDefaultHttpClient(
    IHttpClientFactory httpClientFactory,
    IUserAgentConfiguration? userAgentConfiguration = null,
    int timeoutSeconds = 30)
{
    var httpClient = httpClientFactory.CreateClient();
    // ...
}

All existing call sites in McpToolServerConfigurationService already provide the factory via dependency injection.

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