fix(auth): Decouple CreateRequest interface from UpdateRequest

[jonathanedey]

By inheriting UpdateRequest, CreateRequest unintentionally exposes type definitions and optional nullable contracts designed strictly for user update requests. This results in misleading and incompatible type definitions.

This PR explicitly defines independent property fields on the CreateRequest interface rather than extending UpdateRequest. This resolves misleading and incompatible type inheritance between user creation and user update workflows by:

• Removing providerToLink and providersToUnlink property fields from CreateRequest.
• Removing null type from the displayName, photoURL, and phoneNumber property fields in CreateRequest.

Fixes: #2450, #2929, #3152
Supersedes and closes: #2930

[gemini-code-assist]

Code Review

This pull request refactors the CreateRequest interface in both etc/firebase-admin.auth.api.md and src/auth/auth-config.ts so that it no longer extends UpdateRequest. Instead, the interface now explicitly defines its own properties, including disabled, displayName, email, emailVerified, password, phoneNumber, and photoURL. There are no review comments to evaluate, and I have no additional feedback to provide.

[lahirumaramba]

Thanks! LGTM!