fix(bitpay): align refund declaration with implementation

The Bitpay connector currently advertises refund support in the GET /feature_matrix response, but the refund execution path is not implemented.

• Declaration (refunds marked Supported): https://github.com/juspay/hyperswitch/blob/9e5dd70d1cb4011bbcca3114862406008a0b61f8/crates/hyperswitch_connectors/src/connectors/bitpay.rs#L442-L446

• Implementation (Refund execute returns NotImplemented): https://github.com/juspay/hyperswitch/blob/9e5dd70d1cb4011bbcca3114862406008a0b61f8/crates/hyperswitch_connectors/src/connectors/bitpay.rs#L367-L375

Please either implement the Bitpay refund flow or update the feature matrix declaration to mark refunds as NotSupported.