fix(parquet): propagate nonce generation failures (#1045)

## What changed

Check the secure-random read used to generate Parquet AES nonces and
stop encryption if nonce generation fails.

## Why

The encryptor previously ignored the result of `crypto/rand.Read`. On Go
versions where that function returns an error, encryption could continue
with an all-zero or partially filled nonce. Reusing a nonce undermines
the security guarantees of both AES-GCM and AES-CTR.

The test uses a package-local random-read seam to inject an entropy
failure without replacing the process-wide random reader. It covers both
supported cipher modes.

## Validation

`go test ./parquet/internal/encryption`

Author: fallintoplace

parquet/internal/encryption/aes.go

@@ -43,6 +43,8 @@ const (
 	bufferSizeLength = 4
 )
 
+var readRandom = rand.Read
+
 // Module constants for constructing the AAD bytes, the order here is
 // important as the constants are set via iota.
 const (
@@ -125,7 +127,9 @@ func (a *aesEncryptor) Encrypt(w io.Writer, src, key, aad []byte) int {
 	}
 
 	nonce := make([]byte, NonceLength)
-	rand.Read(nonce)
+	if _, err := readRandom(nonce); err != nil {
+		panic(fmt.Errorf("parquet: failed to generate encryption nonce: %w", err))
+	}
 
 	if a.mode == gcmMode {
 		aead, err := cipher.NewGCM(block)

parquet/internal/encryption/aes_test.go

@@ -19,12 +19,42 @@ package encryption
 import (
 	"bytes"
 	"encoding/binary"
+	"errors"
 	"testing"
 
 	"github.com/apache/arrow-go/v18/parquet"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
+type failingRandomReader struct {
+	err error
+}
+
+func (r failingRandomReader) Read([]byte) (int, error) { return 0, r.err }
+
+func TestAESEncryptRejectsNonceGenerationFailure(t *testing.T) {
+	originalReadRandom := readRandom
+	t.Cleanup(func() { readRandom = originalReadRandom })
+	failingReader := failingRandomReader{err: errors.New("entropy unavailable")}
+	readRandom = failingReader.Read
+
+	for _, tc := range []struct {
+		name string
+		alg  parquet.Cipher
+	}{
+		{name: "GCM", alg: parquet.AesGcm},
+		{name: "CTR", alg: parquet.AesCtr},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			encryptor := NewAesEncryptor(tc.alg, false)
+			require.PanicsWithError(t,
+				"parquet: failed to generate encryption nonce: entropy unavailable",
+				func() { encryptor.Encrypt(&bytes.Buffer{}, []byte("data"), make([]byte, 16), nil) })
+		})
+	}
+}
+
 func TestAESDecryptRejectsMalformedCiphertext(t *testing.T) {
 	decryptor := newAesDecryptor(parquet.AesGcm, false)
 	key := make([]byte, 16)