test(audience-sdk): centralise HTTP test response body and exception fixtures

HttpTransportTests had inline literals for the malformed-body
diagnostic ("not-json"), the empty JSON object body ("{}"), and the
exception messages thrown by MockHandler factories ("connection
refused", "Request timed out", "simulated").

Adds five file-local consts (MalformedResponseBody, EmptyJsonObjectBody,
ConnectionRefusedMessage, RequestTimedOutMessage,
SimulatedCancellationMessage) at the top of the fixture and migrates
six call sites. The empty body "" is left inline since "the body is
truly empty" carries the meaning at the call site.

NUnit assertions never check the exception messages, but pinning the
"what failure was simulated" intent keeps the fixture's catalogue of
network failure modes visible in one place.

Per the user's "everything random goes in a constant" stance.

Follow-up to SDK-272 (centralisation of duplicated literals).

Author: ImmutableJeffrey

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

@@ -17,6 +17,15 @@ namespace Immutable.Audience.Tests
     [TestFixture]
     internal class HttpTransportTests
     {
+        // Response body fixtures.
+        private const string MalformedResponseBody = "not-json";
+        private const string EmptyJsonObjectBody = "{}";
+
+        // Exception messages thrown by MockHandler factories.
+        private const string ConnectionRefusedMessage = "connection refused";
+        private const string RequestTimedOutMessage = "Request timed out";
+        private const string SimulatedCancellationMessage = "simulated";
+
         private string _testDir = null!;
         private DiskStore _store = null!;
 
@@ -176,7 +185,7 @@ public async Task SendBatchAsync_BaseUrlOverride_WinsOverKeyPrefix()
         [Test]
         public async Task SendBatchAsync_EmptyQueue_ReturnsFalse()
         {
-            var handler = new MockHandler(HttpStatusCode.OK, "{}");
+            var handler = new MockHandler(HttpStatusCode.OK, EmptyJsonObjectBody);
             using var transport = new HttpTransport(_store, TestDefaults.PublishableKey, handler: handler);
 
             var sent = await transport.SendBatchAsync();
@@ -359,7 +368,7 @@ public async Task SendBatchAsync_200_MalformedBody_TreatsAsZeroRejected()
             // Malformed diagnostic body must not block the success path.
             _store.Write(WireFixture.Track((MessageFields.EventName, TestEventNames.PlaceholderA)));
 
-            var handler = new MockHandler(HttpStatusCode.OK, "not-json");
+            var handler = new MockHandler(HttpStatusCode.OK, MalformedResponseBody);
             AudienceError? reportedError = null;
             using var transport = new HttpTransport(_store, TestDefaults.PublishableKey,
                 onError: e => reportedError = e, handler: handler);
@@ -491,7 +500,7 @@ public async Task SendBatchAsync_NetworkError_KeepsFilesAndBacksOff()
         {
             _store.Write(WireFixture.Track());
 
-            var handler = new MockHandler(() => throw new HttpRequestException("connection refused"));
+            var handler = new MockHandler(() => throw new HttpRequestException(ConnectionRefusedMessage));
             AudienceError? reportedError = null;
             using var transport = new HttpTransport(_store, TestDefaults.PublishableKey,
                 onError: e => reportedError = e, handler: handler);
@@ -514,7 +523,7 @@ public async Task SendBatchAsync_HttpClientTimeout_TreatedAsNetworkError()
             // NetworkError path.
             _store.Write(WireFixture.Track());
 
-            var handler = new MockHandler(() => throw new TaskCanceledException("Request timed out"));
+            var handler = new MockHandler(() => throw new TaskCanceledException(RequestTimedOutMessage));
             AudienceError? reportedError = null;
             using var transport = new HttpTransport(_store, TestDefaults.PublishableKey,
                 onError: e => reportedError = e, handler: handler);
@@ -540,7 +549,7 @@ public async Task SendBatchAsync_CallerCancelled_Throws_DoesNotDeleteOrRecordFai
             // forever: nothing ever drains, nothing ever throws.
             _store.Write(WireFixture.Track());
 
-            var handler = new MockHandler(() => throw new OperationCanceledException("simulated"));
+            var handler = new MockHandler(() => throw new OperationCanceledException(SimulatedCancellationMessage));
             AudienceError? reportedError = null;
             using var transport = new HttpTransport(_store, TestDefaults.PublishableKey,
                 onError: e => reportedError = e, handler: handler);