ChangeFeed tests: Pins deterministic mode for default-path assertions

Agent-Logs-Url: https://github.com/Azure/azure-cosmos-dotnet-v3/sessions/2ecc1c73-658a-4358-ad20-9ecc87fb25fe

Co-authored-by: kirankumarkolli <6880899+kirankumarkolli@users.noreply.github.com>

Author: Copilot

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/ChangeFeed/DocumentServiceLeaseManagerCosmosTests.cs

@@ -77,6 +77,8 @@ public async Task AcquireCompletes()
         [DataRow(2, DisplayName = "Container with partitionKey PK")]
         public async Task CreatesEPKBasedLease(int factoryType)
         {
+            Environment.SetEnvironmentVariable(ConfigurationManager.ChangeFeedLeaseIdAsPartitionKeyEnabled, "true");
+
             RequestOptionsFactory requestOptionsFactory = GetRequestOptionsFactory(factoryType);
             string continuation = Guid.NewGuid().ToString();
             DocumentServiceLeaseStoreManagerOptions options = new DocumentServiceLeaseStoreManagerOptions
@@ -119,6 +121,8 @@ public async Task CreatesEPKBasedLease(int factoryType)
         [DataRow(2, DisplayName = "Container with partitionKey PK")]
         public async Task CreatesPartitionKeyBasedLease(int factoryType)
         {
+            Environment.SetEnvironmentVariable(ConfigurationManager.ChangeFeedLeaseIdAsPartitionKeyEnabled, "true");
+
             RequestOptionsFactory requestOptionsFactory = GetRequestOptionsFactory(factoryType);
             string continuation = Guid.NewGuid().ToString();
             DocumentServiceLeaseStoreManagerOptions options = new DocumentServiceLeaseStoreManagerOptions