fix(auth): add password strength validation to admin.createUser

[bharabhi01]

The admin.createUser endpoint wasn't checking password strength against configured rules, while admin.updateUser was doing this validation. This creates a security gap where users created via the admin API could have weak passwords that don't meet the configured requirements.

This commit adds the missing password strength validation to ensure consistent security across all user creation paths.

What kind of change does this PR introduce?

This is a security-related bug fix that closes a gap in password validation. It maintains backward compatibility for all valid use cases while enforcing proper security standards for passwords created through the admin API.

What is the current behavior?

Currently, there's an inconsistency in password validation within the Supabase Auth API:

1. The admin.updateUser function correctly validates passwords against configured strength rules
2. The admin.createUser function does not validate passwords against these same rules

This inconsistency creates a security vulnerability where:

1. Users created through the admin API can have weak passwords that don't meet the configured security requirements
2. These weak passwords bypass the password strength rules defined in the application configuration
3. There's an inconsistent security posture between user creation and user updates

As noted in issue #1959, this means that even if an application has strict password rules configured, an administrator can inadvertently create users with weak passwords that would otherwise be rejected if created through normal registration or updated later.

What is the new behavior?

1. The admin.createUser function now validates passwords against the configured strength rules
2. Password validation is consistent across both user creation and user updates
3. Weak passwords are rejected adequately with appropriate error messages (422 Unprocessable Entity with error_code="weak_password")
4. The security gap is closed, ensuring all passwords in the system meet the configured security requirements

Checklist for Submitting Pull Requests

Is there a corresponding issue created for it? If so, please include it in the PR description so we can track/refer to it.:
Fixes #1959

Does your PR follow the semantic-release commit guidelines?:
Yes

Are the existing tests passing?:
The full test suite (make test) shows failures, but these failures existed before my changes and are unrelated to the password validation functionality I've fixed. I've verified that my specific changes work by running the relevant tests in isolation.
To verify my changes, I ran: go test ./internal/api -run TestAdmin -v which confirms the password validation is now working correctly.

Have you written some tests for your PR?:
Yes

[kangmingtay]

@bharabhi01 thanks for contributing! i think we would also want this check to be present in adminUserUpdate and generateLink with the option to bypass the password strength check requirement by passing in an argument of some sort

some folks use the admin create endpoint to create users with arbitrary random passwords, then inviting them and asking them to update their password on first login

[bharabhi01]

Got it! I'll make the required changes to bypass the password strength using an argument wherever necessary.