Guard help article tenancy schema

[swilla]

Summary

• add a shared tenancy helper that checks whether the configured tenant column actually exists before applying tenant-aware behavior
• guard admin, frontend, guest, model scope, model auto-assignment, and factory tenant behavior against missing tenant columns
• add regression coverage for the runtime guard when tenancy is enabled but the table does not have the tenant column

Why

Promote Synergy hit a paid-member Help Center 500 because tenancy scoping queried help_articles.team_id, but the existing table did not have that column. For greenfield installs, the existing create_help_articles_table migration already adds the tenant column when tenancy is enabled before migrations run. This PR keeps that greenfield migration path as-is and focuses on preventing config/schema drift from becoming a runtime 500.

Testing

• php -d memory_limit=-1 vendor/bin/pest --compact tests/Unit/TenancyTest.php tests/Feature/FrontendHelpTest.php tests/Feature/AdminHelpArticleResourceTest.php
• vendor/bin/pint --dirty --test

Notes

• This intentionally does not add a follow-up repair migration. The package README already documents that tenancy must be enabled before running migrations; late tenancy enablement remains a manual database change.