feat: send X-Axme-Client header on every request (#32)

Adds X-Axme-Client: axme-sdk-java/<version> header to all outgoing HTTP
requests, enabling AXME platform admin analytics to identify which SDK
generated the traffic.

- New SDK_VERSION public constant on AxmeClient (kept in sync with releases)
- Header added in the main HttpRequest builder
- Tests verify the header is set on register-nick and check-nick paths

Author: George-iam

src/main/java/dev/axme/sdk/AxmeClient.java

@@ -17,6 +17,13 @@
 import java.util.UUID;
 
 public final class AxmeClient {
+
+  /**
+   * Published axme-sdk-java version. Sent in X-Axme-Client header so AXME
+   * platform analytics can identify SDK usage. Bump on every release.
+   */
+  public static final String SDK_VERSION = "0.2.0";
+
   private final String baseUrl;
   private final String apiKey;
   private final String actorToken;
@@ -864,6 +871,7 @@ Map<String, Object> requestJson(
             .header("Accept", "application/json");
 
     builder.header("x-api-key", apiKey);
+    builder.header("X-Axme-Client", "axme-sdk-java/" + SDK_VERSION);
     String resolvedAuthorization = options.getAuthorization();
     if (isBlank(resolvedAuthorization) && !isBlank(actorToken)) {
       resolvedAuthorization = "Bearer " + actorToken;

src/test/java/dev/axme/sdk/AxmeClientTest.java

@@ -46,6 +46,7 @@ void registerNickSendsPayloadAndHeaders() throws Exception {
     assertEquals("POST", request.getMethod());
     assertEquals("/v1/users/register-nick", request.getPath());
     assertEquals("token", request.getHeader("x-api-key"));
+    assertEquals("axme-sdk-java/" + AxmeClient.SDK_VERSION, request.getHeader("X-Axme-Client"));
     assertEquals("register-1", request.getHeader("Idempotency-Key"));
 
     Map<String, Object> body =
@@ -54,6 +55,14 @@ void registerNickSendsPayloadAndHeaders() throws Exception {
     assertTrue((Boolean) response.get("ok"));
   }
 
+  @Test
+  void clientSendsXAxmeClientHeader() throws Exception {
+    server.enqueue(new MockResponse().setResponseCode(200).setBody("{\"ok\":true,\"available\":true}"));
+    client.checkNick("@partner.user", RequestOptions.none());
+    RecordedRequest request = server.takeRequest();
+    assertEquals("axme-sdk-java/" + AxmeClient.SDK_VERSION, request.getHeader("X-Axme-Client"));
+  }
+
   @Test
   void clientSendsConfiguredActorToken() throws Exception {
     server.enqueue(new MockResponse().setResponseCode(200).setBody("{\"ok\":true,\"available\":true}"));