test(audience-sdk): cover IdentityTypeExtensions.ParseLowercaseString

ParseLowercaseString was added in the SDK-272 stack as the inverse of
ToLowercaseString and consumed by the sample app to map wire strings
back to the enum. It had no direct test, so a typo in any of the eight
case branches or in the Custom fallback would land silently.

Add three parametrised cases:
- Each known enum value maps from its lowercase wire form.
- Mixed-case ("Steam", "STEAM", "Passport") still resolves
  via ToLowerInvariant, matching the documented behaviour.
- null, empty, and unknown values fall back to Custom (the parser
  never throws so producer code always gets a usable enum).

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

Author: ImmutableJeffrey

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

@@ -19,6 +19,37 @@ public void ToLowercaseString_MapsEachEnumValueToLowercaseBackendString(Identity
             Assert.AreEqual(expected, type.ToLowercaseString());
         }
 
+        [TestCase("passport", IdentityType.Passport)]
+        [TestCase("steam", IdentityType.Steam)]
+        [TestCase("epic", IdentityType.Epic)]
+        [TestCase("google", IdentityType.Google)]
+        [TestCase("apple", IdentityType.Apple)]
+        [TestCase("discord", IdentityType.Discord)]
+        [TestCase("email", IdentityType.Email)]
+        [TestCase("custom", IdentityType.Custom)]
+        public void ParseLowercaseString_MapsKnownStringToEnum(string wire, IdentityType expected)
+        {
+            Assert.AreEqual(expected, IdentityTypeExtensions.ParseLowercaseString(wire));
+        }
+
+        [TestCase("Steam", IdentityType.Steam)]
+        [TestCase("STEAM", IdentityType.Steam)]
+        [TestCase("Passport", IdentityType.Passport)]
+        public void ParseLowercaseString_AcceptsMixedCase(string wire, IdentityType expected)
+        {
+            Assert.AreEqual(expected, IdentityTypeExtensions.ParseLowercaseString(wire));
+        }
+
+        [TestCase(null)]
+        [TestCase("")]
+        [TestCase("unknown_provider")]
+        [TestCase("steamX")]
+        public void ParseLowercaseString_FallsBackToCustomForUnknownOrEmpty(string? wire)
+        {
+            // ParseLowercaseString never throws; unknown values map to Custom.
+            Assert.AreEqual(IdentityType.Custom, IdentityTypeExtensions.ParseLowercaseString(wire));
+        }
+
         [Test]
         public void ToLowercaseString_UnknownValue_Throws()
         {