fix: improve consistency and clarity in Simple Domain Allow List POST_LOGIN template

[snocchy]

Changes

This PR improves code consistency and clarity in simple-domain-allowlist-POST_LOGIN template:

• Remove misleading comments about api.access.deny() taking a second userMessage parameter. This parameter applies to pre-user-registration Actions trigger, not post-login.
  • Actions Triggers: post-login - API Object
  • Actions Triggers: pre-user-registration - API Object
• Add missing early return statement for consistency. If the lack of return was intentional, please let me know.
• Separate combined return api.access.deny() statements into distinct lines to clarify the intent of each operation. The return value of onExecutePostLogin seems to have no meaning according to the sample codes in the documentation.
  • Login Trigger

References

• I added at least one link to explain why this change is needed.

We discussed this internally, so I don’t have a public link. I thought it would be faster to just write the code and open a PR rather than start a public discussion.

Testing

Steps to Reproduce

1. Test login attempts with users having:
   • Valid and verified email address from allowed domain
   • Valid and verified email address from non-allowed domain
   • Valid and unverified email address from allowed domain
   • Invalid email address
   • Missing email address
2. Confirm that the denial message is displayed and the action execution stops properly for each invalid case

Checklist

• I have manually tested any new or updated actions templates in a real auth0 account.
• This pull request's title matches the conventional commit format
• This branch is up to date with main
• All existing and new checks complete without errors
• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• I have read the Contribution guide for this repository