Cython Avro decoder reads past the buffer end on malformed input

Two places in `pyiceberg/avro/decoder_fast.pyx` advance the read pointer using a value taken from the stream, without bounding it against `self._end`.

**1. `read_bytes` does not validate the decoded length**

```python
cpdef inline bytes read_bytes(self):
cdef uint64_t length;
if self._current >= self._end: # only confirms 1 byte is available
raise EOFError(f"EOF: read 1 bytes")

decode_zigzag_ints(&self._current, 1, &length)

if length <= 0:
return b""
cdef const unsigned char *r = self._current
self._current += length # not checked against self._end
return r[0:length]
```

The guard confirms one byte is available before decoding the length, but the decoded `length` is then used to slice and to advance `_current` with no check that `_current + length <= _end`. A length field larger than the remaining buffer reads beyond it.

**2. `decode_zigzag_ints` has no end pointer to bound against**

```c
void decode_zigzag_ints(const unsigned char **buffer, const uint64_t count, uint64_t *result);
```

The signature takes a buffer and a count but no end, so the varint walk cannot stop at the buffer boundary — a run of bytes with the continuation bit set keeps advancing. It is called from five sites in the decoder (lines 93, 101, 111, 124, 176), including from `read_bytes` above.

Both are reachable from a malformed or hostile Avro manifest.

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Issue investigation generated via claude, reviewed by Sung, Kevin, Fokko.

pyiceberg/avro/decoder_fast.pyx から始め、read_bytes と decode_zigzag_ints、および issue に記載されている 5 つの呼び出し箇所を読みます。デコーダーを通して、形式が不正または切り詰められた Avro 入力を試します。形式が不正な入力がバッファーの末尾を越えて進めず、有効なデコードが既存のデコーダーの動作によって引き続きカバーされていれば完了です。

索引モデルが issue の本文から書いたものです。