Use duckdb_appender_error_data for appender error handling

Switch the appender from duckdb_appender_error (plain string) to
duckdb_appender_error_data, which exposes both the message and the
DuckDB error type. Route all appender error sites through the existing
DuckDBErrorData.ThrowOnError extension (message prefix now optional),
so appender exceptions carry ErrorType, consistent with the Arrow paths.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LWKN1SKSTFDyeqSrA4dfYT

Author: Giorgi

DuckDB.NET.Bindings/NativeMethods/NativeMethods.Appender.cs

@@ -23,11 +23,9 @@ public static partial class Appender
         [UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
         public static partial DuckDBLogicalType DuckDBAppenderColumnType(DuckDBAppender appender, ulong index);
 
-        [SuppressGCTransition]
-        [LibraryImport(DuckDbLibrary, EntryPoint = "duckdb_appender_error")]
+        [LibraryImport(DuckDbLibrary, EntryPoint = "duckdb_appender_error_data")]
         [UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
-        [return: MarshalUsing(typeof(DuckDBOwnedStringMarshaller))]
-        public static partial string DuckDBAppenderError(DuckDBAppender appender);
+        public static partial DuckDBErrorData DuckDBAppenderErrorData(DuckDBAppender appender);
 
         [LibraryImport(DuckDbLibrary, EntryPoint = "duckdb_appender_flush")]
         [UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]

DuckDB.NET.Data/DuckDBAppender.cs

@@ -1,5 +1,6 @@
 using DuckDB.NET.Data.Common;
 using DuckDB.NET.Data.DataChunk.Writer;
+using DuckDB.NET.Data.Extensions;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 
@@ -72,7 +73,7 @@ public void Clear()
         var state = NativeMethods.Appender.DuckDBAppenderClear(nativeAppender);
         if (!state.IsSuccess())
         {
-            ThrowLastError(nativeAppender);
+            NativeMethods.Appender.DuckDBAppenderErrorData(nativeAppender).ThrowOnError();
         }
 
         rowCount = 0;
@@ -102,7 +103,7 @@ public void Close()
             var state = NativeMethods.Appender.DuckDBAppenderClose(nativeAppender);
             if (!state.IsSuccess())
             {
-                ThrowLastError(nativeAppender);
+                NativeMethods.Appender.DuckDBAppenderErrorData(nativeAppender).ThrowOnError();
             }
         }
         finally
@@ -137,18 +138,9 @@ private void AppendDataChunk()
 
         if (!state.IsSuccess())
         {
-            ThrowLastError(nativeAppender);
+            NativeMethods.Appender.DuckDBAppenderErrorData(nativeAppender).ThrowOnError();
         }
 
         NativeMethods.DataChunks.DuckDBDataChunkReset(dataChunk);
     }
-
-    [DoesNotReturn]
-    [StackTraceHidden]
-    internal static void ThrowLastError(Native.DuckDBAppender appender)
-    {
-        var errorMessage = NativeMethods.Appender.DuckDBAppenderError(appender);
-
-        throw new DuckDBException(errorMessage);
-    }
 }

DuckDB.NET.Data/DuckDBAppenderRow.cs

@@ -135,7 +135,7 @@ public IDuckDBAppenderRow AppendDefault()
 
         if (state == DuckDBState.Error)
         {
-            DuckDBAppender.ThrowLastError(nativeAppender);
+            NativeMethods.Appender.DuckDBAppenderErrorData(nativeAppender).ThrowOnError();
         }
 
         columnIndex++;

DuckDB.NET.Data/DuckDBConnection.cs

@@ -161,7 +161,7 @@ public DuckDBAppender CreateAppender(string? catalog, string? schema, string tab
         {
             try
             {
-                DuckDBAppender.ThrowLastError(nativeAppender);
+                NativeMethods.Appender.DuckDBAppenderErrorData(nativeAppender).ThrowOnError();
             }
             finally
             {

DuckDB.NET.Data/Extensions/DuckDBErrorDataExtensions.cs

@@ -2,14 +2,14 @@ namespace DuckDB.NET.Data.Extensions;
 
 internal static class DuckDBErrorDataExtensions
 {
-    public static void ThrowOnError(this DuckDBErrorData errorData, string message)
+    public static void ThrowOnError(this DuckDBErrorData errorData, string message = "")
     {
         using (errorData)
         {
             if (errorData.HasError)
             {
                 var errorType = NativeMethods.ErrorData.DuckDBErrorDataErrorType(errorData);
-                throw new DuckDBException($"{message} {errorData.Message}".TrimEnd(), errorType);
+                throw new DuckDBException($"{message} {errorData.Message}".Trim(), errorType);
             }
         }
     }