test(tenancy): add tenant data isolation tests for Issue #857

[shogun444]

Closes #857

Summary

This PR adds a dedicated tenant isolation test suite to validate the existing multi-tenant isolation layer.

Changes

• Added src/tenancy/tests/tenant-isolation.spec.ts
• Added IsolationService tests covering tenant context management
• Added TenantRlsSubscriber read isolation tests
• Added TenantRlsSubscriber write protection tests
• Verified cross-tenant writes throw ForbiddenOperationException
• Verified automatic tenantId assignment during inserts
• Covered same-tenant access, missing tenant context, and entities without a tenantId

Notes

• No production code changes
• No database schema changes
• No new dependencies
• No CI changes
• Existing tenant isolation behavior is verified through unit tests using mocked repositories

Known Limitation

Course, Enrollment, and Notification entities currently do not define a tenantId field, so they are not protected by the existing RLS subscriber. This PR documents the current behavior but intentionally does not introduce schema or service changes to keep the scope limited to the requested test suite.

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