test(audience-sdk): centralise Retry-After HTTP header name

HttpTransportTests had "Retry-After" inline three times (across the
delta-seconds, http-date, and past-date 429 tests) and ConsentSyncTests
had it once in CapturingHandler.SendAsync. The string is the
RFC 7231 standard header name; the SDK reads it via the typed
response.Headers.RetryAfter accessor, but the mocks set it by name.

Adds a file-local RetryAfterHeader const at the top of each fixture
(matching the existing publishable-key / response-body file-local
const pattern) and migrates all four sites.

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/ConsentSyncTests.cs

@@ -12,6 +12,10 @@ namespace Immutable.Audience.Tests
     [TestFixture]
     internal class ConsentSyncTests
     {
+        // Standard HTTP header name (RFC 7231) the mock CapturingHandler sets
+        // on 429 responses to override the SDK's default backoff.
+        private const string RetryAfterHeader = "Retry-After";
+
         private string _testDir;
 
         [SetUp]
@@ -223,7 +227,7 @@ protected override async Task<HttpResponseMessage> SendAsync(
                 var response = new HttpResponseMessage(status);
                 if ((int)status == 429 && RetryAfterSeconds.HasValue)
                 {
-                    response.Headers.Add("Retry-After", RetryAfterSeconds.Value.ToString());
+                    response.Headers.Add(RetryAfterHeader, RetryAfterSeconds.Value.ToString());
                 }
                 return response;
             }

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

@@ -20,6 +20,9 @@ internal class HttpTransportTests
         // Non-test-prefix publishable key. Must not carry pk_imapik-test- (asserts production BaseUrl).
         private const string ProdPublishableKey = "pk_imapik-prodkey";
 
+        // Standard HTTP header (RFC 7231) the server sets to override the SDK's 429 backoff.
+        private const string RetryAfterHeader = "Retry-After";
+
         // Response body fixtures.
         private const string MalformedResponseBody = "not-json";
         private const string EmptyJsonObjectBody = "{}";
@@ -241,7 +244,7 @@ public async Task SendBatchAsync_429_RetryAfterDeltaSeconds_OverridesExpoBackoff
             var handler = new MockHandler(() =>
             {
                 var resp = new HttpResponseMessage((HttpStatusCode)429);
-                resp.Headers.Add("Retry-After", "12");
+                resp.Headers.Add(RetryAfterHeader, "12");
                 return resp;
             });
             using var transport = new HttpTransport(_store, TestDefaults.PublishableKey,
@@ -264,7 +267,7 @@ public async Task SendBatchAsync_429_RetryAfterHttpDate_OverridesExpoBackoff()
             var handler = new MockHandler(() =>
             {
                 var resp = new HttpResponseMessage((HttpStatusCode)429);
-                resp.Headers.Add("Retry-After", DateTimeOffset.UtcNow.AddSeconds(20).ToString("R"));
+                resp.Headers.Add(RetryAfterHeader, DateTimeOffset.UtcNow.AddSeconds(20).ToString("R"));
                 return resp;
             });
             using var transport = new HttpTransport(_store, TestDefaults.PublishableKey,
@@ -286,7 +289,7 @@ public async Task SendBatchAsync_429_PastRetryAfterDate_FallsBackToExpoBackoff()
             var handler = new MockHandler(() =>
             {
                 var resp = new HttpResponseMessage((HttpStatusCode)429);
-                resp.Headers.Add("Retry-After", DateTimeOffset.UtcNow.AddSeconds(-30).ToString("R"));
+                resp.Headers.Add(RetryAfterHeader, DateTimeOffset.UtcNow.AddSeconds(-30).ToString("R"));
                 return resp;
             });
             using var transport = new HttpTransport(_store, TestDefaults.PublishableKey,