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

clients/resolvespec-dart/lib/src/client.dart

@@ -0,0 +1,84 @@
+import 'dart:convert';
+
+import 'package:http/http.dart' as http;
+
+import 'types.dart';
+
+/// Thrown on a non-2xx response or an unsuccessful API result.
+class ResolveSpecException implements Exception {
+  final int statusCode;
+  final String message;
+  final ApiError error;
+
+  ResolveSpecException(this.message, this.statusCode, [ApiError? error]) : error = error ?? ApiError(message: message);
+
+  @override
+  String toString() => 'ResolveSpecException($statusCode): $message';
+}
+
+/// Shared HTTP configuration for both clients.
+class ClientOptions {
+  final String? token;
+  final Map<String, String> headers;
+  final Duration timeout;
+
+  /// Supply your own client (tests, pooling).
+  final http.Client? httpClient;
+
+  const ClientOptions({this.token, this.headers = const {}, this.timeout = const Duration(seconds: 30), this.httpClient});
+}
+
+class Transport {
+  final String baseUrl;
+  final ClientOptions options;
+  final http.Client _http;
+
+  Transport(String baseUrl, ClientOptions? options)
+      : baseUrl = baseUrl.replaceAll(RegExp(r'/+$'), ''),
+        options = options ?? const ClientOptions(),
+        _http = options?.httpClient ?? http.Client();
+
+  /// Content-Type < custom headers < per-call headers < bearer token.
+  Future<http.Response> send(String method, Uri uri, {String? body, Map<String, String>? extra}) {
+    final headers = <String, String>{'Content-Type': 'application/json'};
+    void merge(Map<String, String> src) {
+      for (final e in src.entries) {
+        headers.removeWhere((k, _) => k.toLowerCase() == e.key.toLowerCase());
+        headers[e.key] = e.value;
+      }
+    }
+
+    merge(options.headers);
+    if (extra != null) merge(extra);
+    final token = options.token;
+    if (token != null && token.isNotEmpty) merge({'Authorization': 'Bearer $token'});
+
+    final req = http.Request(method, uri)..headers.addAll(headers);
+    if (body != null) req.body = body;
+    return _http.send(req).timeout(options.timeout).then(http.Response.fromStream);
+  }
+
+  void close() => _http.close();
+
+  static ResolveSpecException errorFrom(http.Response resp) {
+    final body = utf8.decode(resp.bodyBytes, allowMalformed: true);
+    ApiError? err;
+    var isJson = false;
+    try {
+      final parsed = jsonDecode(body);
+      isJson = true;
+      if (parsed is Map<String, dynamic> && parsed['error'] is Map<String, dynamic>) {
+        err = ApiError.fromJson(parsed['error'] as Map<String, dynamic>);
+      }
+    } on FormatException {
+      // not JSON
+    }
+    var message = err?.message ?? '';
+    if (message.isEmpty) {
+      var text = isJson ? '' : body.trim();
+      if (text.length > 200) text = text.substring(0, 200);
+      message = text.isNotEmpty ? text : '${resp.reasonPhrase ?? 'Error'} (${resp.statusCode})';
+    }
+    return ResolveSpecException(message, resp.statusCode, err);
+  }
+}