test(audience-sdk): centralise LogTests inputs and reference Log.Prefix

LogTests had inline literals for the Log.Debug / Log.Warn inputs
("silent", "hello", "something off"), the prefix substring asserted
on emitted lines ("[ImmutableAudience]"), and the warn marker
("WARN").

Adds four file-local consts (SilentDebugInput, EnabledDebugInput,
WarnInput, WarnMarker) at the top of the fixture and migrates the
six call sites. The prefix assertion now references Log.Prefix, the
SDK's existing const for the same string, so a SDK-side prefix
rename automatically propagates.

The "WARN" substring is kept as a local marker rather than slicing
Log.WarnPrefix; "[ImmutableAudience] WARN:" includes punctuation
the assertion does not require. A direct WARN constant matches the
test's actual intent ("the warn pipeline includes WARN somewhere").

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/Utility/LogTests.cs

@@ -6,6 +6,14 @@ namespace Immutable.Audience.Tests
     [TestFixture]
     internal class LogTests
     {
+        // Inputs to Log.Debug / Log.Warn used across the fixture.
+        private const string SilentDebugInput = "silent";
+        private const string EnabledDebugInput = "hello";
+        private const string WarnInput = "something off";
+
+        // Substring marker that Log.Warn injects between the prefix and the user message.
+        private const string WarnMarker = "WARN";
+
         private List<string> _captured;
 
         [SetUp]
@@ -28,7 +36,7 @@ public void Debug_WhenDisabled_EmitsNothing()
         {
             Log.Enabled = false;
 
-            Log.Debug("silent");
+            Log.Debug(SilentDebugInput);
 
             Assert.AreEqual(0, _captured.Count);
         }
@@ -38,23 +46,23 @@ public void Debug_WhenEnabled_EmitsWithPrefix()
         {
             Log.Enabled = true;
 
-            Log.Debug("hello");
+            Log.Debug(EnabledDebugInput);
 
             Assert.AreEqual(1, _captured.Count);
-            StringAssert.StartsWith("[ImmutableAudience]", _captured[0]);
-            StringAssert.Contains("hello", _captured[0]);
+            StringAssert.StartsWith(Log.Prefix, _captured[0]);
+            StringAssert.Contains(EnabledDebugInput, _captured[0]);
         }
 
         [Test]
         public void Warn_AlwaysEmits_EvenWhenDisabled()
         {
             Log.Enabled = false;
 
-            Log.Warn("something off");
+            Log.Warn(WarnInput);
 
             Assert.AreEqual(1, _captured.Count);
-            StringAssert.Contains("WARN", _captured[0]);
-            StringAssert.Contains("something off", _captured[0]);
+            StringAssert.Contains(WarnMarker, _captured[0]);
+            StringAssert.Contains(WarnInput, _captured[0]);
         }
     }
 }