fix(python): wrap malformed feed API errors

[floze-the-genius]

Summary

Keep Python FeedClient error handling aligned with the TypeScript SDK when a proxy or upstream returns a non-JSON error body. Malformed JSON and invalid UTF-8 now fall back to the HTTP reason while still raising the typed PmxtError.

Fixes #1700.

Testing

• regression test: malformed HTML body produces PmxtError with the HTTP status/reason and preserves the HTTPError cause
• .venv/bin/pytest tests/test_feed_client.py -q - 2 passed
• .venv/bin/pytest tests -q - 268 passed, 66 deselected
• .venv/bin/black --check pmxt/feed_client.py tests/test_feed_client.py
• generated the Python OpenAPI client with OpenAPI Generator 7.18.0 before installation/tests
• wheel and sdist build; twine check dist/* passed
• git diff --check

Project-wide Mypy is not claimed: the current configuration targets unsupported Python 3.8 under the installed Mypy and reports 118 existing SDK errors, including one pre-existing no-any-return in feed_client.py:102.

AI Assistance Disclosure

OpenAI Codex assisted with issue investigation, implementation, regression-test creation, and validation. I reviewed the final diff and test results.