test(audience-sdk): parametrise DistributionPlatform casing across all platforms

Init's lowercase-normalisation tests covered "Steam" / "STEAM" / "steam"
but not the other four public DistributionPlatforms values (Epic, GOG,
Itch, Standalone). Adding a sixth platform would land without coverage.

Add a parametrised Init_LowercasesDistributionPlatform_AcrossAllPublicValues
test that takes each public DistributionPlatforms constant, uppercases
it, runs Init, and asserts the canonical lowercase form is restored.

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

Author: ImmutableJeffrey

src/Packages/Audience/Tests/Runtime/ImmutableAudienceTests.cs

@@ -1151,6 +1151,21 @@ public void Init_LeavesDistributionPlatformUnchanged_WhenAlreadyLowercase()
             Assert.AreEqual(DistributionPlatforms.Steam, config.DistributionPlatform);
         }
 
+        // Lowercase normalisation must apply to every DistributionPlatforms value.
+        [TestCase(DistributionPlatforms.Steam)]
+        [TestCase(DistributionPlatforms.Epic)]
+        [TestCase(DistributionPlatforms.GOG)]
+        [TestCase(DistributionPlatforms.Itch)]
+        [TestCase(DistributionPlatforms.Standalone)]
+        public void Init_LowercasesDistributionPlatform_AcrossAllPublicValues(string canonical)
+        {
+            var config = MakeConfig();
+            config.DistributionPlatform = canonical.ToUpperInvariant();
+            ImmutableAudience.Init(config);
+
+            Assert.AreEqual(canonical, config.DistributionPlatform);
+        }
+
         [Test]
         public void Init_LeavesDistributionPlatformNull_WhenNotSet()
         {