Registration should require fullName to match User model

[tristanwork]

Parent bounty: #743

Bug

The Prisma User model requires fullName, but the registration schema only accepts email, password, and role. registerUser() also omits fullName from the returned user payload. When the auth flow is wired to Prisma, registration payloads accepted by the API will be missing a required user field.

Expected

• Registration payload validation requires a non-empty fullName.
• registerUser() carries the validated fullName into the returned user payload.
• Tests cover rejection of missing fullName and preservation of a valid name.

Scope

Keep the fix limited to registration validation/service behavior and focused API tests.

[Daisuke134]

/attempt #2770

[mkcash]

I'll work on this fix and submit a PR. Payment info: USDC (BNB Chain): 0xAC1d857cf2A69Ef2C0698F100a644bfE12A6123d / PayPal: ljwtitan@hotmail.com