Add failing test; fix BufferedStreamSource bug where .Position is que…

…ried on non-seekable streams, causing a failure.

src/Imageflow/Fluent/BufferedStreamSource.cs

@@ -8,7 +8,7 @@ public sealed class BufferedStreamSource : IAsyncMemorySource, IMemorySource
 {
     private BufferedStreamSource(Stream stream, bool disposeUnderlying, bool seekToStart)
     {
-        if (stream.Position != 0 && !stream.CanSeek && seekToStart)
+        if (!stream.CanSeek && seekToStart)
         {
             throw new ArgumentException("Stream must be seekable if seekToStart is true");
         }

tests/Imageflow.Test/NonSeekableReadStream.cs

@@ -0,0 +1,62 @@
+namespace Imageflow.Test;
+
+using System;
+using System.IO;
+
+public class NonSeekableReadStream : Stream
+{
+    private byte[] data;
+    private long position = 0; // Current position within the data
+
+    public NonSeekableReadStream(byte[] dataSource)
+    {
+        data = dataSource ?? throw new ArgumentNullException(nameof(dataSource));
+    }
+
+    public override bool CanRead => true;
+    public override bool CanSeek => false;
+    public override bool CanWrite => false;
+
+    public override long Length => data.Length;
+
+    public override long Position
+    {
+        get => throw new NotSupportedException("Seeking not supported.");
+        set => throw new NotSupportedException("Seeking not supported.");
+    }
+
+    public override void Flush()
+    {
+        // No-op for read-only stream
+    }
+
+    public override int Read(byte[] buffer, int offset, int count)
+    {
+        if (buffer == null) throw new ArgumentNullException(nameof(buffer));
+        if (offset < 0 || count < 0) throw new ArgumentOutOfRangeException("offset or count is negative.");
+        if (buffer.Length - offset < count) throw new ArgumentException("The sum of offset and count is greater than the buffer length.");
+
+        int available = data.Length - (int)position;
+        if (available <= 0) return 0; // End of stream
+
+        int toCopy = Math.Min(available, count);
+        Array.Copy(data, position, buffer, offset, toCopy);
+        position += toCopy;
+        return toCopy;
+    }
+
+    public override long Seek(long offset, SeekOrigin origin)
+    {
+        throw new NotSupportedException("Seeking not supported.");
+    }
+
+    public override void SetLength(long value)
+    {
+        throw new NotSupportedException("Setting length not supported.");
+    }
+
+    public override void Write(byte[] buffer, int offset, int count)
+    {
+        throw new NotSupportedException("Writing not supported.");
+    }
+}

tests/Imageflow.Test/TestApi.cs

@@ -332,7 +332,7 @@ public async Task TestBuildCommandStringWithStreamsAndWatermarksLegacy()
             var stream1 = new BufferedStream(new MemoryStream(imageBytes));
             Assert.Equal(137, stream1.ReadByte());
             stream1.Seek(0, SeekOrigin.Begin);
-            var stream2 = new BufferedStream(new MemoryStream(imageBytes));
+            var stream2 = new NonSeekableReadStream(imageBytes);
             var stream3 = new BufferedStream(new MemoryStream(imageBytes));
             using (var b = new ImageJob())
             {
@@ -361,12 +361,12 @@ public async Task TestBuildCommandStringWithStreamsAndWatermarks()
             var stream1 = new BufferedStream(new MemoryStream(imageBytes));
             Assert.Equal(137, stream1.ReadByte());
             stream1.Seek(0, SeekOrigin.Begin);
-            var stream2 = new BufferedStream(new MemoryStream(imageBytes));
+            var stream2 = new NonSeekableReadStream(imageBytes);
             var stream3 = new BufferedStream(new MemoryStream(imageBytes));
             using var b = new ImageJob();
             var watermarks = new List<InputWatermark>();
             watermarks.Add(new InputWatermark(BufferedStreamSource.UseEntireStreamAndDisposeWithSource(stream1), new WatermarkOptions()));
-            watermarks.Add(new InputWatermark(BufferedStreamSource.UseEntireStreamAndDisposeWithSource(stream2), new WatermarkOptions().SetGravity(new ConstraintGravity(100, 100))));
+            watermarks.Add(new InputWatermark(BufferedStreamSource.UseStreamRemainderAndDisposeWithSource(stream2), new WatermarkOptions().SetGravity(new ConstraintGravity(100, 100))));
 
             var r = await b.BuildCommandString(
                 BufferedStreamSource.UseEntireStreamAndDisposeWithSource(stream3),