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?

Start by reproducing the supplied Python example and read the register_codec callbacks together with the asarray block configuration. Compare the input and output sizes for the final block, then determine how the codec should handle that partial block; done means the example completes without the broadcast error.

Written by the indexing model from the issue text.