Raise MessageParseError when a message field is not a dict

[eeshsaxena]

parse_message documents that it raises MessageParseError on bad input, but a user or assistant message whose message field is not a dict leaks a raw TypeError:

from claude_agent_sdk._internal.message_parser import parse_message
parse_message({"type": "user", "message": "oops"})
# TypeError: string indices must be integers, not 'str'

Both handlers read data["message"]["content"] assuming message is an object, so a string, list, or number there blows up before the except KeyError can wrap it. I validate message is a dict up front and raise MessageParseError, the same way the content blocks are already checked (this matches the existing test_non_dict_content_block_raises_documented_error case). Added a parametrized test.

[eeshsaxena]

Hi! Gentle nudge on this one whenever you have some bandwidth. It's a small, self-contained fix (Raise MessageParseError when a message field is not a dict), and it's currently mergeable with no conflicts. No urgency at all, and I'm happy to make any changes you'd like. Thanks for maintaining claude-agent-sdk-python!