test(audience-sdk): share mixed-case wire-format fixtures

Renames TestFixtures.DistributionPlatformSteamCased and DistributionPlatformSteamUppercase to
neutral SteamPascalCase and SteamUpperCase. The same "Steam" / "STEAM" string fixtures are relevant
to two test families: ImmutableAudienceTests' DistributionPlatform lowercase normalisation, and
IdentityTypeTests' ParseLowercaseString case-insensitive matching. The new names drop the
family-specific prefix.

Adds:
- PassportPascalCase ("Passport") for the IdentityType mixed-case fixture batch.
- SteamSuffixed ("steamX") for the steam-prefix-but-not-exact-match fixture in the Custom-fallback
  test.

Migrates references:
- ImmutableAudienceTests:1127 / 1138 (rename only)
- IdentityTypeTests:55-57 (ParseLowercaseString_AcceptsMixedCase cases)
- IdentityTypeTests:66 ([TestCase("steamX")])
- ImmutableAudienceTests:398 (missed inline "user1", now GenericUserSingleId from the earlier
  centralisation pass)

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/IdentityTypeTests.cs

@@ -46,9 +46,9 @@ public void IdentityTypeWireFormat_PinsExactStringValues()
             Assert.AreEqual("custom", IdentityTypeWireFormat.Custom);
         }
 
-        [TestCase("Steam", IdentityType.Steam)]
-        [TestCase("STEAM", IdentityType.Steam)]
-        [TestCase("Passport", IdentityType.Passport)]
+        [TestCase(TestFixtures.SteamPascalCase, IdentityType.Steam)]
+        [TestCase(TestFixtures.SteamUpperCase, IdentityType.Steam)]
+        [TestCase(TestFixtures.PassportPascalCase, IdentityType.Passport)]
         public void ParseLowercaseString_AcceptsMixedCase(string wire, IdentityType expected)
         {
             Assert.AreEqual(expected, IdentityTypeExtensions.ParseLowercaseString(wire));
@@ -57,7 +57,7 @@ public void ParseLowercaseString_AcceptsMixedCase(string wire, IdentityType expe
         [TestCase(null)]
         [TestCase("")]
         [TestCase(TestFixtures.UnknownProvider)]
-        [TestCase("steamX")]
+        [TestCase(TestFixtures.SteamSuffixed)]
         public void ParseLowercaseString_FallsBackToCustomForUnknownOrEmpty(string? wire)
         {
             // ParseLowercaseString never throws; unknown values map to Custom.

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

@@ -395,7 +395,7 @@ public void Identify_InvalidIdentityTypeCast_Throws()
             var invalid = (IdentityType)999;
 
             Assert.Throws<ArgumentOutOfRangeException>(
-                () => ImmutableAudience.Identify("user1", invalid),
+                () => ImmutableAudience.Identify(TestFixtures.GenericUserSingleId, invalid),
                 "invalid enum cast must throw so a broken call fails loud rather than " +
                 "shipping an identify event that cannot be matched for deletion");
         }
@@ -1124,7 +1124,7 @@ public void Init_GameLaunch_IncludesDistributionPlatform()
         public void Init_LowercasesDistributionPlatform_WhenCallerPassesMixedCase()
         {
             var config = MakeConfig();
-            config.DistributionPlatform = TestFixtures.DistributionPlatformSteamCased;
+            config.DistributionPlatform = TestFixtures.SteamPascalCase;
             ImmutableAudience.Init(config);
 
             Assert.AreEqual(DistributionPlatforms.Steam, config.DistributionPlatform,
@@ -1135,7 +1135,7 @@ public void Init_LowercasesDistributionPlatform_WhenCallerPassesMixedCase()
         public void Init_LowercasesDistributionPlatform_WhenCallerPassesAllUpperCase()
         {
             var config = MakeConfig();
-            config.DistributionPlatform = TestFixtures.DistributionPlatformSteamUppercase;
+            config.DistributionPlatform = TestFixtures.SteamUpperCase;
             ImmutableAudience.Init(config);
 
             Assert.AreEqual(DistributionPlatforms.Steam, config.DistributionPlatform);

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

@@ -80,10 +80,13 @@ internal static class TestFixtures
         // File body that occupies the queue directory path so directory creation fails.
         internal const string DiskBlockerContent = "blocker";
 
-        // DistributionPlatform mixed-case fixtures for Init's lowercase
-        // normalisation test.
-        internal const string DistributionPlatformSteamCased = "Steam";
-        internal const string DistributionPlatformSteamUppercase = "STEAM";
+        // Mixed-case "Steam" / "Passport" fixtures shared by DistributionPlatform and IdentityType tests.
+        internal const string SteamPascalCase = "Steam";
+        internal const string SteamUpperCase = "STEAM";
+        internal const string PassportPascalCase = "Passport";
+
+        // Steam-prefixed-but-not-exact-match fixture for the Custom-fallback test.
+        internal const string SteamSuffixed = "steamX";
 
         // Unity Application.platform string for the GameLaunch.Platform test.
         internal const string PlatformWindows = "WindowsPlayer";