test(audience-sdk): centralise EventQueueTests Msg helper key

EventQueueTests' local Msg helper labelled its dict's event-name
field with an inline "event" key. The string is distinct from the
SDK's wire-format MessageFields.EventName ("eventName"); EventQueue
accepts arbitrary dicts so the test could pick anything. Inline made
this implicit.

Adds a file-local MsgEventKey const at the top of the fixture with a
comment noting the test-scaffolding role and migrates the one usage.

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/EventQueueTests.cs

@@ -9,6 +9,9 @@ namespace Immutable.Audience.Tests
     [TestFixture]
     internal class EventQueueTests
     {
+        // Test-scaffold key used by the local Msg helper, not MessageFields.EventName.
+        private const string MsgEventKey = "event";
+
         private string _testDir;
         private DiskStore _store;
 
@@ -28,7 +31,7 @@ public void TearDown()
         }
 
         private static Dictionary<string, object> Msg(string evt) =>
-            new Dictionary<string, object> { ["event"] = evt };
+            new Dictionary<string, object> { [MsgEventKey] = evt };
 
         [Test]
         public void Enqueue_ThenFlushSync_PersistesEventToDisk()