MessageBuffer.putByteBuffer copies incorrect bytes or writes to the wrong positions for sliced and read-only sources in the default Unsafe-backed implementation.

Three valid-input cases:

1. Sliced heap source: wrap [9, 1, 2, 3], set its position to 1, and pass source.slice() with length 3. The destination receives [9, 1, 2] instead of [1, 2, 3], because the source array offset is omitted.
2. Read-only heap source into a sliced heap destination: the fallback writes at index in the backing array, ignoring the destination slice's array offset. Bytes before the destination slice can be overwritten.
3. Read-only heap source into a direct destination: every loop iteration writes at address + index, so only the last source byte survives at that position.

A minimal example for the first case:

ByteBuffer source = ByteBuffer.wrap(new byte[] {9, 1, 2, 3});
source.position(1);
MessageBuffer destination = MessageBuffer.allocate(3);
destination.putByteBuffer(0, source.slice(), 3);
System.out.println(Arrays.toString(destination.toByteArray()));
// Expected: [1, 2, 3]
// Actual:   [9, 1, 2]

Reproduced on current main with OpenJDK 21 on Linux x86_64. A regression matrix covering heap/direct storage, read-only sources, source/destination slices, and zero/partial lengths fails in 10 of 64 cases before the fix. It also checks source position, limit and mark, and destination bytes outside the copied range.