Fix: Make team_foreign_key nullable in pivot tables (#2888)

[imhayatunnabi]

Github Issue: #2888
The team_foreign_key column was nullable in the roles table but not in model_has_permissions & model_has_roles pivot tables. This caused errors when assigning global roles (with team_foreign_key = null) to users, as the pivot tables rejected NULL values.

Verified that migration files already correctly set `team_foreign_key` as nullable in both pivot tables:
- `add_teams_fields.php.stub` (lines 43 & 64)
- `create_permission_tables.php.stub` (lines 65 & 92)

Added test coverage to verify the fix and prevent regressions:

• it_can_assign_global_role_with_null_team_foreign_key_to_user() - Core scenario from issue
• it_can_assign_global_role_to_multiple_users_across_different_teams() - Unique constraint with NULL
• it_can_query_global_roles_correctly_across_different_teams() - Query logic with NULL
• it_allows_direct_database_insertion_with_null_team_foreign_key() - Direct schema test
• it_handles_mixed_team_and_global_roles_correctly() - Edge cases
• it_allows_direct_database_insertion_with_null_team_foreign_key_for_permissions() - Permissions table