[Proto] Fix ProtoWriter buffer growth for large byte arrays

Adjusts ProtoWriter to account for varint length prefix when growing the buffer before writing large byte arrays. Adds a test to verify correct encoding and decoding of 1MB byte arrays.

Author: Linwenxuan04

Lagrange.Proto.Test/SegmentBufferTest.cs

@@ -1,3 +1,4 @@
+using Lagrange.Proto.Primitives;
 using Lagrange.Proto.Utility;
 
 namespace Lagrange.Proto.Test;
@@ -29,7 +30,28 @@ public void TestCreateReadeOnlyMemory()
         Assert.That(reader.Length, Is.EqualTo(_raw.Length));
         Assert.That(reader.Span.ToArray(), Is.EqualTo(_raw));
     }
-    
+
+    [Test]
+    public void TestProtoWriteLargeByteArray()
+    {
+        var segments = new SegmentBufferWriter();
+        var writer = new ProtoWriter(segments);
+        
+        var random = GC.AllocateUninitializedArray<byte>(1024 * 1024 * 1); // 1 MB
+        Random.Shared.NextBytes(random);
+        
+        writer.EncodeBytes(random.AsSpan());
+        writer.Flush();
+        
+        var writtenData = segments.CreateReadOnlyMemory();
+        var reader = new ProtoReader(writtenData.Span);
+        int readData = reader.DecodeVarInt<int>();
+        var readSpan = reader.CreateSpan(readData);
+        
+        Assert.That(readData, Is.EqualTo(random.Length));
+        Assert.That(readSpan.ToArray(), Is.EqualTo(random));
+    }
+
     [TearDown]
     public void TearDown()
     {

Lagrange.Proto/Primitives/ProtoWriter.cs

@@ -66,7 +66,8 @@ public void EncodeString(ReadOnlySpan<char> str)
     public void EncodeBytes(ReadOnlySpan<byte> bytes)
     {
         int length = bytes.Length;
-        if (_memory.Length - BytesPending < length) Grow(length);
+        int count = ProtoHelper.GetVarIntLength(length);
+        if (_memory.Length - BytesPending < length) Grow(length + count);
 
         EncodeVarInt(length);
         bytes.CopyTo(_memory.Span[BytesPending..]);