When creating OIDC users, the provider was calling UpdateUserRoles
even with empty roles due to the default schema value, causing the
server error "User Role Field is set in the OIDC configuration".

OIDC users should get their roles exclusively from the OIDC provider's
role mapping, not from explicit API calls. This fix:

- Errors if explicit roles are provided for OIDC users
- Skips role assignment entirely for OIDC users
- Provides clear error messaging about OIDC role behavior

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Update the Read function to not populate roles from server response for OIDC users.
This prevents Terraform from detecting config drift when OIDC users have roles
assigned by the OIDC provider but an empty roles list in the Terraform config.

Addresses review comment about config drift in PR #247.

Co-authored-by: angrycub <464492+angrycub@users.noreply.github.com>

Update OIDC user role handling to use cleaner Go style:
- Use negative conditions (loginType != codersdk.LoginTypeOIDC) for better readability
- Simplify comments to be more concise and inline
- Maintain all existing validation logic and functionality

Co-authored-by: angrycub <464492+angrycub@users.noreply.github.com>

Fix formatting issues found by go fmt, specifically the closing brace
placement in the ImportState function.

Co-authored-by: angrycub <464492+angrycub@users.noreply.github.com>