feat: add User-Agent header with SDK version

Implements User-Agent header in all HTTP requests with format:
flagsmith-flutter-sdk/<version>

The version is automatically updated by release-please via the
extra-files configuration.

Closes #82

Co-authored-by: Claude <claude@anthropic.com>

Author: emyller

lib/src/flagsmith_client.dart

@@ -8,6 +8,7 @@ import 'package:flutter_client_sse/flutter_client_sse.dart';
 import 'package:rxdart/subjects.dart';
 
 import '../flagsmith.dart';
+import 'version.dart';
 
 /// Flagsmith client initialization
 ///
@@ -233,6 +234,7 @@ class FlagsmithClient {
     var dio = Dio(config.clientOptions)
       ..options.headers[authHeader] = apiKey
       ..options.headers[acceptHeader] = applicationJson
+      ..options.headers[userAgentHeader] = getUserAgent()
       ..options.followRedirects = true;
 
     if (config.isDebug) {

lib/src/version.dart

@@ -0,0 +1,14 @@
+// This file contains the SDK version information
+// The version is automatically updated by release-please
+
+/// The SDK version
+// x-release-please-start-version
+const String sdkVersion = '6.0.3';
+// x-release-please-end
+
+/// Gets the User-Agent header value for the SDK
+///
+/// Format: flagsmith-flutter-sdk/<version>
+String getUserAgent() {
+  return 'flagsmith-flutter-sdk/$sdkVersion';
+}

release-please-config.json

@@ -8,7 +8,11 @@
             "bump-patch-for-minor-pre-major": false,
             "draft": false,
             "prerelease": false,
-            "include-component-in-tag": false
+            "include-component-in-tag": false,
+            "extra-files": [
+                "lib/src/version.dart",
+                "test/fg/flagsmith_user_agent_test.dart"
+            ]
         }
     },
     "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",

test/fg/flagsmith_user_agent_test.dart

@@ -0,0 +1,33 @@
+import 'package:flagsmith/flagsmith.dart';
+import 'package:test/test.dart';
+import '../shared.dart';
+
+void main() {
+  late FlagsmithClient fs;
+
+  group('[User-Agent Header]', () {
+    setUp(() async {
+      fs = await setupClientAdapter(
+        StorageType.inMemory,
+        isDebug: false,
+      );
+    });
+
+    tearDown(() {
+      fs.close();
+    });
+
+    test('Should set User-Agent header with correct SDK version', () {
+      // x-release-please-start-version
+      final expectedVersion = '6.0.3';
+      // x-release-please-end
+      
+      final userAgent = fs.client.options.headers[FlagsmithClient.userAgentHeader];
+      final expectedUserAgent = 'flagsmith-flutter-sdk/$expectedVersion';
+      
+      expect(userAgent, isNotNull, reason: 'User-Agent header should be set');
+      expect(userAgent, equals(expectedUserAgent), 
+        reason: 'User-Agent should be flagsmith-flutter-sdk/$expectedVersion');
+    });
+  });
+}