Raise MessageParseError when the message field is not a dict

[shashvat-singham]

Problem

parse_message wraps every malformation in its own MessageParseError — non-dict input, missing type, missing required fields. But a "message" field that isn't a dict escapes as a bare TypeError:

>>> parse_message({"type": "user", "message": "hi"})
TypeError: string indices must be integers, not 'str'

>>> parse_message({"type": "assistant", "message": "hi"})
TypeError: string indices must be integers, not 'str'

The user/assistant branches catch only KeyError, so indexing into a non-dict falls through. Since this parser consumes the CLI's stream output, one malformed line surfaces as an unrelated-looking TypeError with no message data attached, instead of the documented MessageParseError (which carries data for debugging). The other branches (result, stream_event, system) happen to be shielded because they use .get() first, which makes the asymmetry easy to trip over.

Change

Catch TypeError/AttributeError alongside the existing KeyError in the user and assistant branches and raise MessageParseError("Malformed <kind> message: ...", data), consistent with every other malformation.

Kept as separate except clauses so the existing "Missing required field" wording for KeyError is unchanged — only the previously-uncaught exceptions get the new "Malformed" wording.

Tests

test_parse_non_dict_message_field covers both branches; it fails on main with the raw TypeError and passes with the change.

$ pytest tests/test_message_parser.py -q
81 passed