fix(gotrue): allow TOTP MFA enrollment without an issuer

[AndroidPoet]

What

GoTrueMFAApi.enroll() no longer requires an issuer when enrolling a TOTP factor. The canonical call now works:

final res = await supabase.auth.mfa.enroll(); // factorType defaults to totp

Why

issuer is documented as optional for TOTP (/// [issuer] : Domain which the user is enrolled with (TOTP only).) and is optional in supabase-js, but the argument validation made it mandatory:

if (factorType == FactorType.totp && issuer != null) {
  body['issuer'] = issuer;
} else if (factorType == FactorType.phone && phone != null) {
  body['phone'] = phone;
} else {
  throw ArgumentError('Invalid arguments, expected an issuer for totp ...');
}

When a TOTP factor is enrolled without an issuer, both if branches are false, so it falls through to the else and throws ArgumentError synchronously — before any request is sent. The standard mfa.enroll() therefore always failed unless an issuer was passed.

This was an unintended regression from #1188 (Add phone mfa enrollment), which introduced the combined branch. The existing enroll totp integration test always passes issuer: 'MyFriend', so it never exercised the no-issuer path.

The fix separates the factor-type handling so TOTP forwards issuer only when provided, while phone enrollment keeps its existing requirement that a phone number is supplied.

Not a breaking change

• TOTP enrollment with an issuer behaves exactly as before (the value is still sent).
• Phone enrollment is unchanged and still throws ArgumentError when phone is omitted (covered by the existing enroll phone requires phone number test).
• Only the previously-broken TOTP-without-issuer path changes: it now succeeds instead of throwing.

Tests

Added packages/gotrue/test/mfa_enroll_test.dart, an offline test using a recording BaseClient that inspects the outgoing request body:

• enroll() reaches the network and sends no issuer (previously threw ArgumentError).
• enroll(issuer: 'MyApp') forwards the issuer.
• enroll(factorType: FactorType.phone) without a phone still throws ArgumentError.