fix: deliver messages whose base64 payload cannot be decoded

`base64.b64decode` raises on a payload that is not valid base64, and the
exception escaped `Message.from_encoded` rather than being reported. RSL6b
asks for the failure to be logged and the message delivered with the
encodings that were not applied, which is what the missing-cipher and
unsupported-encoding branches alongside it already do.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: owenpearson

ably/types/mixins.py

@@ -1,4 +1,5 @@
 import base64
+import binascii
 import json
 import logging
 
@@ -67,8 +68,14 @@ def decode(data, encoding='', cipher=None, context=None):
                     continue
                 data = json.loads(data)
             elif encoding == 'base64':
-                data = bytearray(base64.b64decode(data)) if isinstance(data, bytes) \
-                    else bytearray(base64.b64decode(data.encode('utf-8')))
+                try:
+                    data = bytearray(base64.b64decode(data)) if isinstance(data, bytes) \
+                        else bytearray(base64.b64decode(data.encode('utf-8')))
+                except (binascii.Error, UnicodeEncodeError) as e:
+                    log.error('Message cannot be decoded. '
+                              f'Invalid base64 payload: {e}')
+                    encoding_list.append(encoding)
+                    break
                 if not encoding_list:
                     last_payload = data
             elif encoding == ENC_VCDIFF:

test/unit/message_test.py

@@ -45,3 +45,15 @@ def test_update_inner_message_fields_for_presence_msg_tm2():
         assert presence_msg.get('connectionId') == 'custom_connection_id'
         assert presence_msg.get('timestamp') == 23134
         msg_index = msg_index + 1
+
+
+# RSL6b
+def test_invalid_base64_data_is_delivered_with_remaining_encoding():
+    message = ably.types.message.Message.from_encoded({
+        'name': 'event',
+        'data': 'encrypted-data-here',
+        'encoding': 'custom-encryption/base64',
+    })
+
+    assert message.data == 'encrypted-data-here'
+    assert message.encoding == 'custom-encryption/base64'