Issues with user-defined codecs

I'm facing issues when creating a simple codec that just makes a copy of the data to get familiar with Blosc's registering machinery. I attach the code:

```python
import blosc2
import numpy as np

# Create an User-defined codec (just a memcpy)

def encoder(input, output, meta, schunk: blosc2.SChunk):
print(f"Encoder output size: {output.size}")
output[:schunk.blocksize] = input[:schunk.blocksize]
return schunk.blocksize

def decoder(input, output, meta, schunk: blosc2.SChunk):
output[:schunk.blocksize] = input[:schunk.blocksize]
return schunk.blocksize

# Register the codec
codec_id = 200
blosc2.register_codec('test1', codec_id, encoder, decoder)

# Compress this array with the new codec

shape = (100, 100)
a = np.ones(shape, dtype=np.int64)

cparams = {
'codec': codec_id,
'nthreads': 1,
'filters': [],
'splitmode': blosc2.SplitMode.NEVER_SPLIT,
}

dparams = {
'nthreads': 1,
}

chunks = shape
blocks = (50, 50)

c_a = blosc2.asarray(a, chunks=chunks, blocks=blocks, cparams=cparams, dparams=dparams)
```

However, when I run the previous code, I get the following:
```
Encoder output size: 20000
Encoder output size: 20000
Encoder output size: 20000
Encoder output size: 19968

ValueError: could not broadcast input array from shape (20000,) into shape (19968,)
```

Looking at this, it appears that the last block of the chunk is smaller than the others. Do you know what is happening? Is there something I'm doing wrong?

Empieza reproduciendo el ejemplo de Python proporcionado y lee los callbacks de register_codec junto con la configuración de bloques de asarray. Compara los tamaños de entrada y salida del bloque final y determina después cómo debe manejar el codec ese bloque parcial; se considera terminado cuando el ejemplo se completa sin el error de broadcast.

Escrito por el modelo de indexación a partir del texto del issue.