fix(python): log malformed hosted response bodies at debug

[ThomasHartDev]

Fixes #1792

_json_payload in sdks/python/pmxt/_hosted_errors.py swallowed a ValueError from response.json() and returned None with no logging. When the hosted trading API returned a non-JSON or malformed body, the resulting HostedTradingError lost all detail about the raw response, which makes backend contract changes and malformed error responses harder to diagnose.

This adds a module logger and a logger.debug line in the except path so the parse failure is recorded before falling back to None. Behavior is otherwise unchanged: the function still returns None and the error mapping flow is untouched.

I used %s-style logging to match the stdlib logging already used in the SDK (ws_client.py) rather than the dict-arg form in the issue, since that form doesn't fit stdlib % formatting.

Added a test in tests/test_hosted_errors.py that drives a malformed 5xx body through raise_from_response and asserts the debug record is emitted while the HostedTradingError still raises. Verified locally: the full test_hosted_errors.py suite passes (29 tests), and the new test fails without the log line.