fix: fallout from decimal merge regarding null inference

src/example-basic/Program.cs

@@ -1,19 +1,19 @@
 using System;
 using QuestDB;
 
-using var sender =  Sender.New("http::addr=localhost:9000;");
+using var sender = Sender.New("http::addr=localhost:9000;protocol_version=3");
 await sender.Table("trades")
-    .Symbol("symbol", "ETH-USD")
-    .Symbol("side", "sell")
-    .Column("price", 2615.54)
-    .Column("amount", 0.00044)
-    .AtAsync(DateTime.UtcNow);
+            .Symbol("symbol", "ETH-USD")
+            .Symbol("side", "sell")
+            .Column("price", 2615.54)
+            .Column("amount", 0.00044)
+            .AtAsync(DateTime.UtcNow);
 
 await sender.Table("trades")
-    .Symbol("symbol", "BTC-USD")
-    .Symbol("side", "sell")
-    .Column("price", 39269.98)
-    .Column("amount", 0.001)
-    .AtAsync(DateTime.UtcNow);
+            .Symbol("symbol", "BTC-USD")
+            .Symbol("side", "sell")
+            .Column("price", 39269.98)
+            .Column("amount", 0.001)
+            .AtAsync(DateTime.UtcNow);
 
-await sender.SendAsync();
+await sender.SendAsync();

src/net-questdb-client-tests/HttpTests.cs

@@ -101,7 +101,7 @@ await sender.Table("metrics")
                     .Symbol("tag", "value")
                     .Column("dec_pos", 123.45m)
                     .Column("dec_neg", -123.45m)
-                    .Column("dec_null", (decimal?)null)
+                    .NullableColumn("dec_null", (decimal?)null)
                     .Column("dec_max", decimal.MaxValue)
                     .Column("dec_min", decimal.MinValue)
                     .AtAsync(new DateTime(1970, 01, 01, 0, 0, 1));
@@ -1233,7 +1233,6 @@ public async Task CancelLineAfterError()
         Assert.That(srv.PrintBuffer(), Is.EqualTo(expected));
     }
 
-
     [Test]
     public async Task CannotConnect()
     {

src/net-questdb-client-tests/JsonSpecTestRunner.cs

@@ -45,7 +45,7 @@ public class JsonSpecTestRunner
     private static readonly TestCase[]? TestCases = ReadTestCases();
 
     /// <summary>
-    /// Populate the provided sender with the test case's table, symbols, and columns, then send the prepared row.
+    ///     Populate the provided sender with the test case's table, symbols, and columns, then send the prepared row.
     /// </summary>
     /// <param name="sender">The ISender to configure and use for sending the test case row.</param>
     /// <param name="testCase">The test case containing table name, symbols, and typed columns to write.</param>
@@ -82,13 +82,14 @@ private static async Task ExecuteTestCase(ISender sender, TestCase testCase)
                     var value = ((JsonElement)column.Value).GetString();
                     if (value is null)
                     {
-                        sender.Column(column.Name, (decimal?)null);
+                        sender.NullableColumn(column.Name, (decimal?)null);
                     }
                     else
                     {
                         var d = decimal.Parse(value, NumberStyles.Number, CultureInfo.InvariantCulture);
                         sender.Column(column.Name, d);
                     }
+
                     break;
 
                 default:
@@ -103,9 +104,13 @@ private static async Task ExecuteTestCase(ISender sender, TestCase testCase)
     }
 
     /// <summary>
-    /// Executes the provided test case by sending its configured table, symbols, and columns to a local TCP listener and asserting the listener's received output against the test case's expected result.
+    ///     Executes the provided test case by sending its configured table, symbols, and columns to a local TCP listener and
+    ///     asserting the listener's received output against the test case's expected result.
     /// </summary>
-    /// <param name="testCase">The test case to run; provides table, symbols, columns to send and a Result describing the expected validation (Status, Line, AnyLines, or BinaryBase64).</param>
+    /// <param name="testCase">
+    ///     The test case to run; provides table, symbols, columns to send and a Result describing the
+    ///     expected validation (Status, Line, AnyLines, or BinaryBase64).
+    /// </param>
     [TestCaseSource(nameof(TestCases))]
     public async Task RunTcp(TestCase testCase)
     {
@@ -162,9 +167,13 @@ public async Task RunTcp(TestCase testCase)
     }
 
     /// <summary>
-    /// Executes the provided test case by sending data over HTTP to a dummy server using a QuestDB sender and validates the server's response according to the test case result.
+    ///     Executes the provided test case by sending data over HTTP to a dummy server using a QuestDB sender and validates
+    ///     the server's response according to the test case result.
     /// </summary>
-    /// <param name="testCase">The test case describing table, symbols, columns, and expected result (status, line(s), or base64 binary) to execute and validate.</param>
+    /// <param name="testCase">
+    ///     The test case describing table, symbols, columns, and expected result (status, line(s), or
+    ///     base64 binary) to execute and validate.
+    /// </param>
     [TestCaseSource(nameof(TestCases))]
     public async Task RunHttp(TestCase testCase)
     {
@@ -293,7 +302,7 @@ public class TestCase
         [JsonPropertyName("result")] public TestCaseResult Result { get; set; } = null!;
 
         /// <summary>
-        /// Provides the test case name for display and logging.
+        ///     Provides the test case name for display and logging.
         /// </summary>
         /// <returns>The TestName of the test case.</returns>
         public override string ToString()