[enoki] fix decoding of ephemeral keypair secret (#16104)

## Description 

Describe the changes or additions included in this PR.

## Test Plan 

How did you test the new or updated feature?

---
If your changes are not user-facing and do not break anything, you can
skip the following section. Otherwise, please briefly describe what has
changed under the Release Notes section.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

.changeset/breezy-ties-juggle.md

@@ -0,0 +1,5 @@
+---
+'@mysten/enoki': patch
+---
+
+Fix decoding of ephemeral keypair secret

sdk/enoki/src/EnokiFlow.ts

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 
 import type { SuiClient } from '@mysten/sui.js/client';
+import { decodeSuiPrivateKey } from '@mysten/sui.js/cryptography';
 import { Ed25519Keypair } from '@mysten/sui.js/keypairs/ed25519';
 import type { TransactionBlock } from '@mysten/sui.js/transactions';
 import { fromB64, toB64 } from '@mysten/sui.js/utils';
@@ -160,7 +161,7 @@ export class EnokiFlow {
 			expiresAt: estimatedExpiration,
 			maxEpoch,
 			randomness,
-			ephemeralKeyPair: ephemeralKeyPair.export().privateKey,
+			ephemeralKeyPair: toB64(decodeSuiPrivateKey(ephemeralKeyPair.getSecretKey()).secretKey),
 		});
 
 		return oauthUrl;