Populate the HTTPException body

[afeld]

I was trying to debug 404s and wasn't able to see the HTTP response body — it's always an empty string. I found the relevant code, then had Copilot make the fix. I tried running the test locally, but couldn't figure out how to build with httpfs. Hoping that CI shows it working. I haven't done C++ in 20 years, so open to any suggestions.

Here's Copilot's explanation of the change:

---

HTTPException.body could remain empty because Python exception mapping only read response_body from DuckDB error extra-info. Some HTTP errors now surface payload under body, so the attribute was silently unset.

• Exception field mapping

  • Update HTTP exception translation to accept both extra-info keys:
    • legacy: response_body
    • current/alternate: body
  • This preserves backward compatibility while correctly populating HTTPException.body for newer payloads.

• Test expectation alignment

  • Adjust HTTPFS exception assertion to validate body as a string instead of assuming it is always empty.

} else if (entry.first == "response_body" || entry.first == "body") {
    e.attr("body") = entry.second;
}