This fix addresses an issue where REQ messages with multiple filters
were being rejected by the relay. Notably: The nostrmarket extension's
"Refresh from Nostr" functionality sends a single REQ message containing
4 different filter subscriptions:
- Direct Messages (kinds: [4])
- Stalls (kinds: [30017])
- Products (kinds: [30018])
- Profile (kinds: [0])
Changes:
- Changed validation from `len(data) != 3` to `len(data) < 3` to allow
multiple filters
- Added loop to process all filters in a single REQ message (data[2:])
- Accumulate responses from all filters before returning
This ensures compatibility with clients that batch multiple subscription
filters in a single REQ message, which is a valid pattern according to
NIP-01.
