feat(clients): add Go, Rust, C# and Dart clients for ResolveSpec and FunctionSpec

clients/resolvespec-cs/src/Http.cs

@@ -0,0 +1,78 @@
+using System.Net.Http.Headers;
+using System.Text;
+using System.Text.Json;
+
+namespace ResolveSpec;
+
+/// <summary>Shared HTTP configuration for both clients.</summary>
+public sealed class ClientOptions
+{
+    public string? Token { get; set; }
+    public Dictionary<string, string> Headers { get; } = new(StringComparer.OrdinalIgnoreCase);
+    public TimeSpan Timeout { get; set; } = TimeSpan.FromSeconds(30);
+    /// <summary>Supply your own HttpClient (tests, pooling). Its BaseAddress is ignored.</summary>
+    public HttpClient? HttpClient { get; set; }
+}
+
+internal sealed class Transport
+{
+    public readonly string BaseUrl;
+    readonly ClientOptions _o;
+    readonly HttpClient _http;
+
+    public Transport(string baseUrl, ClientOptions? o)
+    {
+        BaseUrl = baseUrl.TrimEnd('/');
+        _o = o ?? new ClientOptions();
+        _http = _o.HttpClient ?? new HttpClient { Timeout = _o.Timeout };
+    }
+
+    /// <summary>Content-Type &lt; custom headers &lt; per-call headers &lt; bearer token.</summary>
+    public async Task<(HttpResponseMessage resp, string body)> SendAsync(
+        HttpMethod method, string url, string? json, IDictionary<string, string>? extra, CancellationToken ct)
+    {
+        using var req = new HttpRequestMessage(method, url);
+        if (json != null) req.Content = new StringContent(json, Encoding.UTF8, "application/json");
+        foreach (var (k, v) in _o.Headers) Set(req, k, v);
+        if (extra != null) foreach (var (k, v) in extra) Set(req, k, v);
+        if (!string.IsNullOrEmpty(_o.Token)) req.Headers.Authorization = new AuthenticationHeaderValue("Bearer", _o.Token);
+        var resp = await _http.SendAsync(req, ct).ConfigureAwait(false);
+        var body = await resp.Content.ReadAsStringAsync(ct).ConfigureAwait(false);
+        return (resp, body);
+    }
+
+    static void Set(HttpRequestMessage req, string name, string value)
+    {
+        req.Headers.Remove(name);
+        if (!req.Headers.TryAddWithoutValidation(name, value) && req.Content != null)
+        {
+            req.Content.Headers.Remove(name);
+            req.Content.Headers.TryAddWithoutValidation(name, value);
+        }
+    }
+
+    public static ResolveSpecException ErrorFrom(int status, string body, string? reason)
+    {
+        ApiError? err = null;
+        var isJson = false;
+        try
+        {
+            using var doc = JsonDocument.Parse(body);
+            isJson = true;
+            if (doc.RootElement.ValueKind == JsonValueKind.Object && doc.RootElement.TryGetProperty("error", out var e) && e.ValueKind == JsonValueKind.Object)
+                err = e.Deserialize<ApiError>();
+        }
+        catch (JsonException) { }
+
+        var message = err?.Message;
+        if (string.IsNullOrEmpty(message))
+        {
+            var text = isJson ? "" : body.Trim();
+            if (text.Length > 200) text = text[..200];
+            message = text.Length > 0 ? text : $"{reason ?? "Error"} ({status})";
+        }
+        return new ResolveSpecException(message, status, err);
+    }
+
+    public static string Segment(string s) => Uri.EscapeDataString(s);
+}