Browser clients using the Idempotency-Key header for request
deduplication were blocked by CORS preflight because the header
was not listed in Access-Control-Allow-Headers.
Add Idempotency-Key to _CORS_HEADERS and add tests for both the
new header allowance and the existing Vary: Origin behavior.
Co-authored-by: aydnOktay <aydnOktay@users.noreply.github.com>
Co-authored-by: Hermes Agent <hermes@nousresearch.com>