fix: pass through client errors (400/404/409/422) and strip stale item references when store=false

[kiroo92]

Problem

OpenClaw sends POST /v1/responses with input items referencing previous response IDs (rs_...). Since OAuth accounts force store=false, these items are never persisted on OpenAI's side, causing 404 errors. The proxy converts 404 to a generic 502, so the client keeps retrying the same broken request in an infinite loop.

Changes

1. handleErrorResponse: Pass through 400/404/409/422 as invalid_request_error with the original upstream status code and message, instead of converting to generic 502.
2. applyCodexOAuthTransform: When store=false (always true for OAuth accounts), force preserveReferences=false in filterCodexInput to remove item_reference entries and strip stale id fields from input items.

Result

• Input items referencing non-persisted responses are cleaned before forwarding to upstream
• Client errors are passed through transparently so callers can act on the real problem
• Tool call continuation (call_id) still works correctly