fix(enterprise-teams): add ValidateDiagFunc to enterprise_slug and team_slug schema fields

[vmvarela]

Description

Reviewer feedback: move input validation from CRUD functions to schema ValidateDiagFunc so users get errors at plan time instead of apply.

Acceptance Criteria

• Add ValidateDiagFunc: validation.ToDiagFunc(validation.All(validation.StringIsNotWhiteSpace, validation.StringIsNotEmpty)) to enterprise_slug in all resources and data sources
• Add same validation to team_slug fields
• Remove equivalent manual checks from Create/Read/Update/Delete functions
• Verify make build passes

Notes

Upstream PR: integrations#3008