Two speech-services of the same vendor should not be allowed on SP level

Currently, testing in combination with webapp main branch, I am allowed to add two speech service credentials for the same vendor.

**How to test:**
1. Add speech-service for "All accounts"
2. Add speech-service with the same vendor also for "All accounts"

**Result**: you will be allowed to create the second cred of the same vendor
**Expected result:** not allowed to create same vendor twice

On the Account level, it works as expected.


Start by reproducing the speech-service credential creation flow for two credentials with the same vendor and compare it with the account-level validation that already rejects duplicates. Trace the API entry point and validation used for service-provider-level credentials; done means the second same-vendor credential is rejected while different vendors remain allowed.

Written by the indexing model from the issue text.