fix(sdk-core): rollback api-mock.ts to use deprecate fetch client from core

[kpanot]

Proposed change

Rollback api-mock.ts to use deprecate fetch client from core.
This change will be part of #2117.

Related issues

• 🔗 Related to: [Feature]: Turn SDK Mock API to ApiClient agnostic implementation #2117

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[cpaulve-1A]

Should we keep both lines separated?

[kpanot]

I would say yes, it is not leading to any issue and will reduce the diff change when migrating to client-fetch :)