RestClientOptions.UserAgent missing from requests with useClientFactory: true

Describe the bug

When useClientFactory: true is passed to the RestClient constructor and UserAgent is set via RestClientOptions.UserAgent, the User-Agent header is missing from outgoing requests for every RestClient instance after the first one sharing the same BaseUrl.

When disabling the client factory the User-Agent is back on every request.

To Reproduce

var options = new RestClientOptions
 {
     BaseUrl = new Uri("https://httpbin.org"),
     UserAgent = "MyApp/1.0"
 };

//1st request
var client1 = new RestClient(options, useClientFactory: true);
_ = await client1.GetAsync(new RestRequest("/get"));

// 2nd request
var client2 = new RestClient(options, useClientFactory: true);
_ = await client2.GetAsync(new RestRequest("/get"));

1st raw request from fiddler

GET https://httpbin.org/get HTTP/1.1
Host: httpbin.org
User-Agent: MyApp/1.0
Accept: application/json, text/json, text/x-json, text/javascript, application/xml, text/xml
Accept-Encoding: gzip, deflate, br

2nd raw request from fiddler

GET https://httpbin.org/get HTTP/1.1
Host: httpbin.org
Accept: application/json, text/json, text/x-json, text/javascript, application/xml, text/xml
Accept-Encoding: gzip, deflate, br

Expected behavior

User-Agent should appear on every request regardless of how many RestClient instances are created for the same BaseUrl.

Desktop:

• OS: Windows 10.0.26200.8457
• .NET version: .NET 10.0.8
• Version 114.0.0

Additional context

I suspect that ConfigureDefaultParameters should be called regardless of the client instance coming from cache or not.

Start at the RestClient constructor path for useClientFactory: true and inspect ConfigureDefaultParameters when a cached client is reused. Reproduce the two-client example against the same BaseUrl, then verify that UserAgent is present on every outgoing request, including requests from the second instance.