refactor(audience-sdk): introduce AudienceErrorMessages catalogue

Names the message strings routed through AudienceConfig.OnError so a wording tweak touches one file
and runtime emit / test assert read from the same constants.

- Log.cs: adds AudienceErrorMessages with the LocalStorageReadFailed / BatchPartiallyRejected
  formatters and the BatchRejectedPrefix / ServerErrorWillRetryPrefix / ConsentSyncFailedWithStatus
  / ConsentSyncThrew entries.
- HttpTransport.cs: flush onError messages route through AudienceErrorMessages.
- ImmutableAudience.cs: consent-sync onError messages route through AudienceErrorMessages.

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/ImmutableAudience.cs

@@ -656,7 +656,7 @@ private static void SyncConsentToBackend(AudienceConfig config, ConsentLevel lev
                         }
 
                         NotifyErrorCallback(onError, AudienceErrorCode.ConsentSyncFailed,
-                            $"Consent sync failed with status {(int)response.StatusCode}");
+                            AudienceErrorMessages.ConsentSyncFailedWithStatus((int)response.StatusCode));
                         return;
                     }
                 }
@@ -667,7 +667,7 @@ private static void SyncConsentToBackend(AudienceConfig config, ConsentLevel lev
                 catch (Exception ex)
                 {
                     NotifyErrorCallback(onError, AudienceErrorCode.ConsentSyncFailed,
-                        $"Consent sync threw: {ex.Message}");
+                        AudienceErrorMessages.ConsentSyncThrew(ex));
                 }
             });
         }

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

@@ -81,7 +81,7 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
                 // Non-IOException = unrecoverable storage failure (e.g. permissions);
                 // retry won't help. Drop the batch, report via onError.
                 _store.Delete(batch);
-                NotifyError(AudienceErrorCode.FlushFailed, $"Local storage read failed: {ex.Message}");
+                NotifyError(AudienceErrorCode.FlushFailed, AudienceErrorMessages.LocalStorageReadFailed(ex));
                 return true;
             }
 
@@ -122,7 +122,7 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
                     if (rejected > 0)
                     {
                         NotifyError(AudienceErrorCode.ValidationRejected,
-                            $"Batch partially rejected: {rejected} of {batch.Count} events dropped");
+                            AudienceErrorMessages.BatchPartiallyRejected(rejected, batch.Count));
                     }
                 }
                 else if (statusCode == (int)HttpStatusCode.TooManyRequests)
@@ -149,7 +149,7 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
                     _store.Delete(batch);
                     ResetBackoff();
                     NotifyError(AudienceErrorCode.ValidationRejected,
-                        FormatHttpError("Batch rejected", statusCode, rejectionBody));
+                        FormatHttpError(AudienceErrorMessages.BatchRejectedPrefix, statusCode, rejectionBody));
                 }
                 else
                 {
@@ -158,7 +158,7 @@ internal async Task<bool> SendBatchAsync(CancellationToken ct = default)
                     var serverBody = await ReadBodyForErrorAsync(response).ConfigureAwait(false);
                     RecordFailure();
                     NotifyError(AudienceErrorCode.FlushFailed,
-                        FormatHttpError("Server error, will retry", statusCode, serverBody));
+                        FormatHttpError(AudienceErrorMessages.ServerErrorWillRetryPrefix, statusCode, serverBody));
                 }
             }
             catch (OperationCanceledException) when (ct.IsCancellationRequested)

src/Packages/Audience/Runtime/Utility/Log.cs

@@ -43,6 +43,25 @@ private static void Emit(string line)
         }
     }
 
+    // Error messages we pass to AudienceConfig.OnError.
+    internal static class AudienceErrorMessages
+    {
+        internal static string LocalStorageReadFailed(Exception ex) =>
+            $"Local storage read failed: {ex.Message}";
+
+        internal static string BatchPartiallyRejected(int rejected, int total) =>
+            $"Batch partially rejected: {rejected} of {total} events dropped";
+
+        internal const string BatchRejectedPrefix = "Batch rejected";
+        internal const string ServerErrorWillRetryPrefix = "Server error, will retry";
+
+        internal static string ConsentSyncFailedWithStatus(int statusCode) =>
+            $"Consent sync failed with status {statusCode}";
+
+        internal static string ConsentSyncThrew(Exception ex) =>
+            $"Consent sync threw: {ex.Message}";
+    }
+
     internal static class AudienceLogs
     {
         // ---- Init / config validation ----