fix: raise MessageParseError for non-dict message fields in parse_message

[Otis0408]

Summary

parse_message() documents Raises: MessageParseError: If parsing fails, and the maintainers already enforce this one level down: test_parse_assistant_string_content_raises and test_non_dict_content_block_raises_documented_error assert that a malformed content value surfaces as MessageParseError, "never a raw TypeError".

That guard stops at content. When the enclosing object is present but is not a dict, the nested subscript raises a raw TypeError that escapes the except KeyError-only handlers and reaches the caller, violating the documented contract:

input	branch	raised on main
{"type": "user", "message": "hi"}	user	TypeError: string indices must be integers
{"type": "assistant", "message": None}	assistant	TypeError: 'NoneType' object is not subscriptable
{"type": "rate_limit_event", "rate_limit_info": "x", ...}	rate_limit_event	TypeError (info["status"])
{"type": "result", ..., "deferred_tool_use": "oops"}	result	TypeError (deferred["id"])

Because neither public call site wraps parse_message (ClaudeSDKClient.receive_messages() / receive_response() and the query() path), these reach SDK consumers as a raw TypeError, so callers that except MessageParseError (or the exported base ClaudeSDKError) miss them.

Fix

Add isinstance guards mirroring the existing isinstance(raw_content, list) check, so a non-dict message / rate_limit_info / deferred_tool_use raises a descriptive MessageParseError instead of a raw TypeError.

• No happy-path behavior change.
• The except KeyError handlers and their existing messages (asserted by tests) are untouched, so missing-field cases still report "Missing required field ...".
• Guards raise inside the existing try blocks; since MessageParseError is not a KeyError it propagates as documented (same as the current content guard).

Tests

Adds regression tests covering all four branches, parametrized over str / list / None / int message values. Verified they fail on main with the raw TypeError and pass with the fix.

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mypy src: Success: no issues found in 24 source files