[Thin Client Integration]: Adds empty string for RegionalDatabaseAccount parameter while creating RNTBD request. (#5471)

# Pull Request Template

## Description

Starting direct version 3.41.0, we will need to pass empty string for
regionalDatabaseAccountName to serialize the request for proxy.
ThinProxy has support of retrieving regionalDatabaseAccountName when its
empty. Instead of sending globalDatabaseAccountName send empty string.

The new contract in TransportSerialization:
https://msdata.visualstudio.com/CosmosDB/_git/CosmosDB/commit/3a7ef7e72685727de7762dd6769042cd765f7397?path=/Product/Microsoft.Azure.Documents/SharedFiles/Rntbd2/TransportSerialization.cs&version=GBmaster&line=112&lineEnd=113&lineStartColumn=1&lineEndColumn=1&type=2&lineStyle=plain&_a=files

## Type of change

Please delete options that are not relevant.

- [X] Bug fix (non-breaking change which fixes an issue)

## Closing issues

To automatically close an issue: closes #5426

Author: aavasthy

Microsoft.Azure.Cosmos/src/ThinClientTransportSerializer.cs

@@ -113,7 +113,7 @@ public static async Task<Stream> SerializeProxyRequestAsync(
                     activityId,
                     bufferProvider.Provider,
                     accountName,
-                    accountName,
+                    string.Empty,
                     out _,
                     out _);
 

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.EmulatorTests/CosmosItemTests.cs

@@ -275,7 +275,7 @@ public async Task NegativeCreateItemTest(bool binaryEncodingEnabledInClient)
                 {
                     Assert.AreEqual(999999, ce.SubStatusCode);
                     string exception = ce.ToString();
-                    Assert.IsTrue(exception.StartsWith("Microsoft.Azure.Cosmos.CosmosException : Response status code does not indicate success: Forbidden (403); Substatus: 999999; "));
+                    Assert.IsTrue(exception.Contains("Response status code does not indicate success: Forbidden (403); Substatus: 999999; "));
                     string diagnostics = ce.Diagnostics.ToString();
                     Assert.IsTrue(diagnostics.Contains("999999"));
                     CosmosItemTests.ValidateCosmosException(ce);

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.EmulatorTests/CosmosItemThinClientTests.cs

@@ -13,6 +13,7 @@ namespace Microsoft.Azure.Cosmos.SDK.EmulatorTests
     using System.Text.Json;
     using System.Text.Json.Serialization;
     using System.Threading.Tasks;
+    using global::Azure;
     using global::Azure.Core;
     using Microsoft.Azure.Cosmos.Fluent;
     using Microsoft.VisualStudio.TestTools.UnitTesting;
@@ -113,6 +114,76 @@ private async Task<List<TestObject>> CreateItemsSafeAsync(IEnumerable<TestObject
             return itemsCreated;
         }
 
+        [TestMethod]
+        [TestCategory("ThinClient")]
+        public async Task RegionalDatabaseAccountNameIsEmptyInPayload()
+        {
+            byte[] capturedPayload = null;
+            Environment.SetEnvironmentVariable(ConfigurationManager.ThinClientModeEnabled, "True");
+            this.connectionString = Environment.GetEnvironmentVariable("COSMOSDB_THINCLIENT");
+
+            // Initialize the serializer locally
+            JsonSerializerOptions jsonSerializerOptions = new JsonSerializerOptions
+            {
+                PropertyNamingPolicy = null,
+                PropertyNameCaseInsensitive = true,
+                DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
+            };
+            CosmosSystemTextJsonSerializer serializer = new CosmosSystemTextJsonSerializer(jsonSerializerOptions);
+
+            CosmosClientBuilder builder = new CosmosClientBuilder(this.connectionString)
+                .WithConnectionModeGateway()
+                .WithCustomSerializer(serializer)
+                .WithSendingRequestEventArgs(async (sender, e) =>
+                {
+                    if (e.HttpRequest.Version == new Version(2, 0))
+                    {
+                        if (e.HttpRequest.Content != null)
+                        {
+                            capturedPayload = await e.HttpRequest.Content.ReadAsByteArrayAsync();
+                        }
+                    }
+                });
+
+            using CosmosClient client = builder.Build();
+            string uniqueDbName = "TestRegional_" + Guid.NewGuid().ToString();
+            Database database = await client.CreateDatabaseIfNotExistsAsync(uniqueDbName);
+            string uniqueContainerName = "TestRegionalContainer_" + Guid.NewGuid().ToString();
+            Container container = await database.CreateContainerIfNotExistsAsync(uniqueContainerName, "/pk");
+
+            string pk = "pk_regional";
+            TestObject testItem = this.GenerateItems(pk).First();
+
+            // Act
+            ItemResponse<TestObject> response = await container.CreateItemAsync(testItem, new PartitionKey(testItem.Pk));
+            Assert.AreEqual(HttpStatusCode.Created, response.StatusCode);
+
+            // Assert
+            Assert.IsNotNull(capturedPayload, "The request payload was not captured.");
+
+
+            // The RNTBD protocol serializes an empty string as a token with a length of 0.
+            // For `regionalDatabaseAccountName`, which is a SmallString (type 0x02), this is
+            // serialized as two bytes: 0x02 (type) and 0x00 (length).
+            // This byte pair represents an empty string value in RNTBD’s small-string encoding.
+            byte[] emptyStringToken = { 0x02, 0x00 };
+
+            bool foundEmptyStringToken = false;
+            for (int i = 0; i <= capturedPayload.Length - emptyStringToken.Length; i++)
+            {
+                if (capturedPayload[i] == emptyStringToken[0] && capturedPayload[i + 1] == emptyStringToken[1])
+                {
+                    foundEmptyStringToken = true;
+                    break;
+                }
+            }
+
+            Assert.IsTrue(foundEmptyStringToken, "The RNTBD payload should contain a token representing an empty string for the regional account name.");
+
+            // Cleanup
+            await database.DeleteAsync();
+        }
+
         [TestMethod]
         [TestCategory("ThinClient")]
         public async Task HttpRequestVersionIsTwoPointZeroWhenUsingThinClientMode()
@@ -173,12 +244,9 @@ public async Task CreateItemsTest()
         public async Task CreateItemsTestWithThinClientFlagEnabledAndAccountDisabled()
         {
             Environment.SetEnvironmentVariable(ConfigurationManager.ThinClientModeEnabled, "True");
-            string connectionString = ConfigurationManager.GetEnvironmentVariable<string>("COSMOSDB_MULTI_REGION", string.Empty);
-
-            if (string.IsNullOrEmpty(connectionString))
-            {
-                Assert.Fail("Set environment variable COSMOSDB_MULTI_REGION to run the tests");
-            }
+            string authKey = Utils.ConfigurationManager.AppSettings["MasterKey"];
+            string endpoint = Utils.ConfigurationManager.AppSettings["GatewayEndpoint"];
+            AzureKeyCredential masterKeyCredential = new AzureKeyCredential(authKey);
 
             JsonSerializerOptions jsonSerializerOptions = new JsonSerializerOptions
             {
@@ -189,7 +257,8 @@ public async Task CreateItemsTestWithThinClientFlagEnabledAndAccountDisabled()
             this.cosmosSystemTextJsonSerializer = new MultiRegionSetupHelpers.CosmosSystemTextJsonSerializer(jsonSerializerOptions);
 
             this.client = new CosmosClient(
-                  connectionString,
+                  endpoint,
+                  masterKeyCredential,
                   new CosmosClientOptions()
                   {
                       ConnectionMode = ConnectionMode.Gateway,