fix(audience-transport): surface 4xx body + messages envelope

src/Packages/Audience/Runtime/Transport/HttpTransport.cs

@@ -119,10 +119,22 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
                 {
                     // 4xx: server rejected the payload. Drop it (retry won't help) and
                     // reset backoff — server is healthy, our data was the problem.
+                    // Pull the response body so the studio has something actionable:
+                    // the status code alone just says "something is wrong"; the body
+                    // names which field or event broke validation. Truncated to keep
+                    // logs sane if the backend ever returns a long diagnostic.
+                    string? body = null;
+                    try { body = await response.Content.ReadAsStringAsync().ConfigureAwait(false); }
+                    catch { /* Content read failed (disposed, network) — fall through with body=null. */ }
+
                     _store.Delete(batch);
                     ResetBackoff();
-                    NotifyError(AudienceErrorCode.ValidationRejected,
-                        $"Batch rejected with {statusCode}");
+
+                    const int maxBodyLen = 500;
+                    var message = string.IsNullOrEmpty(body)
+                        ? $"Batch rejected with {statusCode}"
+                        : $"Batch rejected with {statusCode}: {(body.Length > maxBodyLen ? body.Substring(0, maxBodyLen) + "…" : body)}";
+                    NotifyError(AudienceErrorCode.ValidationRejected, message);
                 }
                 else
                 {
@@ -210,11 +222,11 @@ private void ResetBackoff()
         }
 
         // Reads each path and wraps the concatenated JSON bodies in
-        // {"batch":[msg1,msg2,...]}. Returns null if every path was
+        // {"messages":[msg1,msg2,...]}. Returns null if every path was
         // unreadable; the caller treats null as "nothing to send".
         private static string? BuildPayload(IReadOnlyList<string> paths)
         {
-            var sb = new StringBuilder("{\"batch\":[");
+            var sb = new StringBuilder("{\"messages\":[");
             var count = 0;
 
             for (var i = 0; i < paths.Count; i++)

src/Packages/Audience/Tests/Runtime/Transport/HttpTransportTests.cs

@@ -87,7 +87,7 @@ public async Task SendBatchAsync_200_SendsGzippedPayloadWithCorrectHeaders()
             Assert.AreEqual("gzip", capturedContentEncoding);
 
             var decompressed = DecompressGzip(capturedBody);
-            StringAssert.StartsWith("{\"batch\":[", decompressed);
+            StringAssert.StartsWith("{\"messages\":[", decompressed);
             StringAssert.EndsWith("]}", decompressed);
             StringAssert.Contains("\"eventName\":\"test\"", decompressed);
         }
@@ -116,7 +116,7 @@ public async Task SendBatchAsync_200_SendsPlainJsonPayloadWithoutContentEncoding
             Assert.AreEqual("pk_imapik-test-key1", capturedKey);
             Assert.AreEqual("application/json", capturedContentType);
             Assert.AreEqual(0, capturedContentEncodingCount, "no Content-Encoding header is permitted in v1");
-            StringAssert.StartsWith("{\"batch\":[", capturedBody);
+            StringAssert.StartsWith("{\"messages\":[", capturedBody);
             StringAssert.EndsWith("]}", capturedBody);
             StringAssert.Contains("\"eventName\":\"test\"", capturedBody);
         }
@@ -186,7 +186,8 @@ public async Task SendBatchAsync_4xx_DeletesFilesAndResetsBackoff()
         {
             _store.Write("{\"type\":\"track\"}");
 
-            var handler = new MockHandler(HttpStatusCode.BadRequest, "");
+            var handler = new MockHandler(HttpStatusCode.BadRequest,
+                "{\"error\":\"invalid eventName format at /batch/0/eventName\"}");
             AudienceError reportedError = null;
             using var transport = new HttpTransport(_store, "pk_imapik-test-key1",
                 onError: e => reportedError = e, handler: handler);
@@ -197,6 +198,9 @@ public async Task SendBatchAsync_4xx_DeletesFilesAndResetsBackoff()
             Assert.IsFalse(transport.IsInBackoffWindow);
             Assert.IsNotNull(reportedError);
             Assert.AreEqual(AudienceErrorCode.ValidationRejected, reportedError.Code);
+            // Backend-supplied diagnostic must reach the studio, otherwise a 400
+            // collapses to "something broke, good luck" with no actionable signal.
+            StringAssert.Contains("invalid eventName format", reportedError.Message);
         }
 
         [Test]

src/Packages/Audience/Tests/Runtime/Utility/GzipTests.cs

@@ -29,7 +29,7 @@ public void Compress_ProducesValidGzip_ThatDecompressesToOriginal()
         public void Compress_OutputIsSmallerThanInput_ForRealisticPayload()
         {
             // Repeated field names compress well in JSON batches.
-            var sb = new StringBuilder("{\"batch\":[");
+            var sb = new StringBuilder("{\"messages\":[");
             for (var i = 0; i < 20; i++)
             {
                 if (i > 0) sb.Append(',');