fix: replace per-call new HttpClient() with shared static instance

[DaraOladapo]

Summary

Closes #36.

Every public method in the library was creating and disposing a fresh HttpClient inside a using block:

// Before — repeated in every method across all 5 files
using (HttpClient httpClient = new HttpClient())
{
    httpClient.DefaultRequestHeaders.Authorization = ...;
    var httpResponse = await httpClient.GetAsync(new Uri(Route));
    return httpResponse;
}

This is the well-documented .NET anti-pattern described in Microsoft's HttpClient guidelines. Each construction opens a new socket; Dispose() does not immediately release it — the OS holds the connection in TIME_WAIT. Under any meaningful request volume this exhausts the available socket pool and causes SocketException.

Fix

Introduced a single private static readonly HttpClient _httpClient in Get.cs (shared across all partial-class files). All active methods now build a per-request HttpRequestMessage and pass it to _httpClient.SendAsync(request):

// After — shared instance, per-request message
private static readonly HttpClient _httpClient = new HttpClient();

public static async Task<HttpResponseMessage> GetAsync(string Route, string Token = null)
{
    var request = new HttpRequestMessage(HttpMethod.Get, new Uri(Route));
    if (!string.IsNullOrEmpty(Token))
        request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", Token);
    return await _httpClient.SendAsync(request);
}

Using HttpRequestMessage is required here: per-call headers (Authorization, custom) are set on the message, not on DefaultRequestHeaders, so concurrent calls cannot race on shared state.

Files changed

File	Change
Get.cs	Added _httpClient static field; all 8 active methods use HttpRequestMessage
Post.cs	All 8 active methods use HttpRequestMessage
Put.cs	All 8 active methods use HttpRequestMessage
Patch.cs	All 8 active methods use HttpRequestMessage
Delete.cs	All 8 active methods use HttpRequestMessage

Non-breaking

Public API signatures are unchanged. The #region TODO commented-out blocks are preserved (tracked separately in #37).

Test plan

• Build passes (dotnet build)
• Existing WireMock integration tests pass (dotnet test)
• Verify no SocketException under concurrent load (manual or load test)

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