test(audience-sdk): centralise SessionTests sabotage message and DiskStoreTests inline JSON keys

SessionTests had "track explode" inline four times as the
InvalidOperationException message thrown by ThrowingTrack delegates
across four session lifecycle scenarios. Adds a TrackExplodeMessage
file-local const at the top of the fixture and migrates all four.

DiskStoreTests built a Purchase-shaped JSON payload by hand-rolling
the wire format with inline keys ("type", "eventName", "anonymousId",
"userId", "properties", "currency", "value") and inline values
("track", "purchase", "USD", "a"). Replaces those with the SDK's
existing constants (MessageFields.*, MessageTypes.Track,
EventNames.Purchase, EventPropertyKeys.Currency / .Value) and the
TestFixtures fixtures already centralised (UsdCurrency,
TestEventNames.PlaceholderA).

The "u" userId placeholder is left inline; centralising a single
one-char placeholder would add more noise than it removes.

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/Core/SessionTests.cs

@@ -10,6 +10,9 @@ namespace Immutable.Audience.Tests
     [TestFixture]
     internal class SessionTests
     {
+        // Exception thrown by the ThrowingTrack sabotage delegate across four scenarios.
+        private const string TrackExplodeMessage = "track explode";
+
         private List<(string name, Dictionary<string, object> props)> _events;
 
         [SetUp]
@@ -546,7 +549,7 @@ public void OnHeartbeat_TrackCallbackThrows_DoesNotEscape()
                 void ThrowingTrack(string name, Dictionary<string, object> props)
                 {
                     if (name == EventNames.SessionHeartbeat)
-                        throw new InvalidOperationException("track explode");
+                        throw new InvalidOperationException(TrackExplodeMessage);
                 }
 
                 using var session = new Session(ThrowingTrack);
@@ -581,7 +584,7 @@ public void Start_TrackCallbackThrows_DoesNotEscape()
                 void ThrowingTrack(string name, Dictionary<string, object> props)
                 {
                     if (name == EventNames.SessionStart)
-                        throw new InvalidOperationException("track explode");
+                        throw new InvalidOperationException(TrackExplodeMessage);
                 }
 
                 using var session = new Session(ThrowingTrack);
@@ -616,7 +619,7 @@ public void End_TrackCallbackThrows_DoesNotEscape()
                 void ThrowingTrack(string name, Dictionary<string, object> props)
                 {
                     if (name == EventNames.SessionEnd)
-                        throw new InvalidOperationException("track explode");
+                        throw new InvalidOperationException(TrackExplodeMessage);
                 }
 
                 using var session = new Session(ThrowingTrack);
@@ -652,7 +655,7 @@ public void SafeTrack_LogWriterThrows_DoesNotEscape()
                 void ThrowingTrack(string name, Dictionary<string, object> props)
                 {
                     if (name == EventNames.SessionHeartbeat)
-                        throw new InvalidOperationException("track explode");
+                        throw new InvalidOperationException(TrackExplodeMessage);
                 }
 
                 using var session = new Session(ThrowingTrack);

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

@@ -207,8 +207,13 @@ public void ApplyAnonymousDowngrade_PurchaseValue_RoundsTripsExactlyForRealistic
                 TearDown();
                 SetUp();
 
-                var json = "{\"type\":\"track\",\"eventName\":\"purchase\",\"anonymousId\":\"a\",\"userId\":\"u\","
-                    + "\"properties\":{\"currency\":\"USD\",\"value\":" + amount + "}}";
+                var json = $"{{\"{MessageFields.Type}\":\"{MessageTypes.Track}\","
+                    + $"\"{MessageFields.EventName}\":\"{EventNames.Purchase}\","
+                    + $"\"{MessageFields.AnonymousId}\":\"{TestEventNames.PlaceholderA}\","
+                    + $"\"{MessageFields.UserId}\":\"u\","
+                    + $"\"{MessageFields.Properties}\":{{"
+                    + $"\"{EventPropertyKeys.Currency}\":\"{TestFixtures.UsdCurrency}\","
+                    + $"\"{EventPropertyKeys.Value}\":{amount}}}}}";
                 _store.Write(json);
 
                 _store.ApplyAnonymousDowngrade();