test(audience-sdk): centralise publishable-key fixtures across transport, stress, and offline tests

Three test files had inline publishable-key fixtures with no
intra-test reason for diverging from the canonical
TestDefaults.PublishableKey:

- ThreadSafetyStressTests:48 used "pk_imapik-test-stress"
- OfflineResilienceTests:47 used "pk_imapik-test-key"

Both fall under "any test-prefix key, content does not matter".
Migrated to TestDefaults.PublishableKey.

HttpTransportTests:162 used "pk_imapik-prodkey" specifically to
verify a non-test-prefix key resolves to ProductionBaseUrl. The
test cannot use TestDefaults.PublishableKey since that one IS
test-prefixed. Added a file-local ProdPublishableKey const
alongside the existing response-body fixtures and migrated the
one usage. The const carries a comment explaining the prefix
constraint.

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/OfflineResilienceTests.cs

@@ -44,7 +44,7 @@ protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage reques
 
         private AudienceConfig MakeConfig() => new AudienceConfig
         {
-            PublishableKey = "pk_imapik-test-key",
+            PublishableKey = TestDefaults.PublishableKey,
             Consent = ConsentLevel.Anonymous,
             PersistentDataPath = _testDir,
             FlushIntervalSeconds = TestDefaults.FlushIntervalSeconds,

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

@@ -45,7 +45,7 @@ protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage reques
         private AudienceConfig MakeConfig(ConsentLevel consent = ConsentLevel.Full) =>
             new AudienceConfig
             {
-                PublishableKey = "pk_imapik-test-stress",
+                PublishableKey = TestDefaults.PublishableKey,
                 Consent = consent,
                 PersistentDataPath = _testDir,
                 FlushIntervalSeconds = TestDefaults.FlushIntervalSeconds,

src/Packages/Audience/Tests/Runtime/Transport/HttpTransportTests.cs

@@ -17,6 +17,9 @@ namespace Immutable.Audience.Tests
     [TestFixture]
     internal class HttpTransportTests
     {
+        // Non-test-prefix publishable key. Must not carry pk_imapik-test- (asserts production BaseUrl).
+        private const string ProdPublishableKey = "pk_imapik-prodkey";
+
         // Response body fixtures.
         private const string MalformedResponseBody = "not-json";
         private const string EmptyJsonObjectBody = "{}";
@@ -156,7 +159,7 @@ public async Task SendBatchAsync_200_UsesCorrectUrlForProdKey()
             HttpRequestMessage? captured = null;
             var handler = new MockHandler(HttpStatusCode.OK, $"{{\"accepted\":1,\"{ResponseFields.Rejected}\":0}}",
                 onRequest: req => captured = req);
-            using var transport = new HttpTransport(_store, "pk_imapik-prodkey", handler: handler);
+            using var transport = new HttpTransport(_store, ProdPublishableKey, handler: handler);
 
             await transport.SendBatchAsync();