Restsharp adds UserAgent headers to HttpClient instance that is passed explicitly

[p-skakun]

Describe the bug
When create multiple RestClient instances with the same HttpClient instance and default options it adds UserAgent header each time.
In certain scenarios it causes System.InvalidOperationException: Collection was modified

To Reproduce

var httpClient = new HttpClient();
var restClient1 = new RestClient(httpClient);
var restClient2 = new RestClient(httpClient);
Assert.AreEqual(2, httpClient.DefaultRequestHeaders.UserAgent.Count);
Assert.AreEqual(httpClient.DefaultRequestHeaders.UserAgent.First(), httpClient.DefaultRequestHeaders.UserAgent.Last());

Expected behavior
Reused httpClient keeps unique UserAgent headers

Desktop (please complete the following information):

• OS: Windows 11 Home
• .NET version: 7
• Version: 109.0.0-preview.1

Additional context
I believe the issues starts here:
https://github.com/restsharp/RestSharp/blob/dev/src/RestSharp/RestClient.cs#L131

In case of default UserAgent value Product?.Name is RestSharp, while UserAgent is RestSharp/x.x.x so the check always passes.

[tacosontitan]

ConfigureHttpClient is not called in the constructor utilized by your reproduction steps. However, this is reproducible:

// arrange
var httpClient = new HttpClient();
var clientOptions = new RestClientOptions();

// act
var restClient1 = new RestClient(httpClient, clientOptions);
var restClient2 = new RestClient(httpClient, clientOptions);

// assert
Assert.Contains(httpClient.DefaultRequestHeaders.UserAgent, agent => $"{agent.Product.Name}/{agent.Product.Version}" == clientOptions.UserAgent);
Assert.Single(httpClient.DefaultRequestHeaders.UserAgent);

In RestClient's current state, this test will fail. This is caused by the fact that the current snippet only compares the Product.Name property with the options.UserAgent which also contains a version.

RestSharp/src/RestSharp/RestClient.cs

Lines 128 to 136 in f7d3984

	void ConfigureHttpClient(HttpClient httpClient) {
	if (Options.MaxTimeout > 0) httpClient.Timeout = TimeSpan.FromMilliseconds(Options.MaxTimeout);
	if (Options.UserAgent != null && httpClient.DefaultRequestHeaders.UserAgent.All(x => x.Product?.Name != Options.UserAgent)) {
	httpClient.DefaultRequestHeaders.TryAddWithoutValidation("User-Agent", Options.UserAgent);
	}
	if (Options.Expect100Continue != null) httpClient.DefaultRequestHeaders.ExpectContinue = Options.Expect100Continue;
	}

Note that line 131 here is the source of the issue.

[alexeyzimarev]

Fixed by @tacosontitan in #2008, I released it in 109.0.1.