[codex] Preserve remote plugin download status errors

[xl-openai]

Summary

• preserve the original HTTP status when a remote plugin bundle download returns a non-success response
• retain at most 8 KiB of the error response body and annotate truncation or body-read failures
• add regression coverage for an oversized error response

Root cause

The non-success response path reused the normal size-limited body reader. When an error response exceeded 8 KiB, that reader returned DownloadTooLarge before the code constructed DownloadStatus, masking the upstream HTTP status and response context.

Impact

Remote plugin installation failures now retain the actionable upstream HTTP status without allowing unbounded error bodies into logs.

Validation

• just test -p codex-app-server plugin_install_preserves_status_when_remote_bundle_error_body_is_too_large
• just fmt
• git diff --check