Codex CLI expects the field name to be `http_headers` (not `headers`)
in the MCP server configuration TOML format.
Changes:
- Update import_from_codex() to read from both `http_headers` (correct)
and `headers` (legacy) with priority to `http_headers`
- Update json_server_to_toml_table() to write `http_headers` instead
of `headers`
- Update core_fields lists to use `http_headers` for HTTP/SSE types
This follows the same pattern as the recent Gemini fix and ensures
backward compatibility while generating correct Codex-compatible configs.
01d8bb53ac