[ivorbosloper]

Two converter improvements that came up in fiboa/cli#345, where the JP converter had to override both.

• request_headers on a converter is sent with every download, next to avoid_range_request. Some servers refuse clients that do not look like a browser.
• Without a cache, the DuckDB converter used to hand every source to DuckDB, which can only read parquet in place. Sources it cannot read, such as archives, are now downloaded to a temporary folder first, as the GeoDataFrame-based converter already does. Remote parquet without a cache is still read in place.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The focused changes address prior feedback, include targeted regression coverage, and have no unresolved blocking findings.

No open issues 🎉

1 resolved since last review

• Merge HTTP headers case-insensitively

_{Review effort: Balanced}