feat(DbExporter): add counting data reader for accurate row count

Wrap IDataReader with CountingDataReader to track rows as they're read
during protobuf serialization, fixing the export returning 0 rows.

Tools/DbExporter/CountingDataReader.cs

@@ -0,0 +1,60 @@
+using System.Data;
+
+namespace DbExporter;
+
+/// <summary>
+/// Wraps an IDataReader to count rows as they're read.
+/// </summary>
+internal sealed class CountingDataReader : IDataReader
+{
+    private readonly IDataReader _inner;
+    private int _rowCount;
+
+    public CountingDataReader(IDataReader inner)
+    {
+        _inner = inner;
+    }
+
+    public int RowCount => _rowCount;
+
+    public bool Read()
+    {
+        var result = _inner.Read();
+        if (result) _rowCount++;
+        return result;
+    }
+
+    // Delegate all other members to inner reader
+    public object this[int i] => _inner[i];
+    public object this[string name] => _inner[name];
+    public int Depth => _inner.Depth;
+    public bool IsClosed => _inner.IsClosed;
+    public int RecordsAffected => _inner.RecordsAffected;
+    public int FieldCount => _inner.FieldCount;
+    public void Close() => _inner.Close();
+    public void Dispose() => _inner.Dispose();
+    public bool GetBoolean(int i) => _inner.GetBoolean(i);
+    public byte GetByte(int i) => _inner.GetByte(i);
+    public long GetBytes(int i, long fieldOffset, byte[]? buffer, int bufferoffset, int length) => _inner.GetBytes(i, fieldOffset, buffer, bufferoffset, length);
+    public char GetChar(int i) => _inner.GetChar(i);
+    public long GetChars(int i, long fieldoffset, char[]? buffer, int bufferoffset, int length) => _inner.GetChars(i, fieldoffset, buffer, bufferoffset, length);
+    public IDataReader GetData(int i) => _inner.GetData(i);
+    public string GetDataTypeName(int i) => _inner.GetDataTypeName(i);
+    public DateTime GetDateTime(int i) => _inner.GetDateTime(i);
+    public decimal GetDecimal(int i) => _inner.GetDecimal(i);
+    public double GetDouble(int i) => _inner.GetDouble(i);
+    public Type GetFieldType(int i) => _inner.GetFieldType(i);
+    public float GetFloat(int i) => _inner.GetFloat(i);
+    public Guid GetGuid(int i) => _inner.GetGuid(i);
+    public short GetInt16(int i) => _inner.GetInt16(i);
+    public int GetInt32(int i) => _inner.GetInt32(i);
+    public long GetInt64(int i) => _inner.GetInt64(i);
+    public string GetName(int i) => _inner.GetName(i);
+    public int GetOrdinal(string name) => _inner.GetOrdinal(name);
+    public DataTable GetSchemaTable() => _inner.GetSchemaTable()!;
+    public string GetString(int i) => _inner.GetString(i);
+    public object GetValue(int i) => _inner.GetValue(i);
+    public int GetValues(object[] values) => _inner.GetValues(values);
+    public bool IsDBNull(int i) => _inner.IsDBNull(i);
+    public bool NextResult() => _inner.NextResult();
+}

Tools/DbExporter/DatabaseExporter.cs

@@ -26,7 +26,8 @@ public sealed class DatabaseExporter
         command.CommandText = definition.Query;
         command.CommandTimeout = 0; // No timeout for large exports
 
-        await using var reader = await command.ExecuteReaderAsync(cancellationToken);
+        await using var baseReader = await command.ExecuteReaderAsync(cancellationToken);
+        var reader = new CountingDataReader(baseReader);
 
         long uncompressedSize = 0;
 
@@ -51,9 +52,7 @@ public sealed class DatabaseExporter
 
         var compressedSize = new FileInfo(definition.OutputPath).Length;
 
-        // Row count requires a separate pass or we estimate from verify
-        // Return 0 for now, verify will get accurate count
-        return new ExportResult(0, uncompressedSize, compressedSize, hash);
+        return new ExportResult(reader.RowCount, uncompressedSize, compressedSize, hash);
     }
 
     private static DbConnection CreateConnection(string providerType, string connectionString)