Write unit and integration tests for TenantSessionStore dual persistence

The rollback test is the trickiest: TenantSessionStore must write to SecureStorage first, then call the Supabase RPC; if the RPC throws, it must call SecureStorage.delete before re-throwing. Test this by setting up the mock to: allow the write, throw on the RPC call, then assert delete was subsequently called. For JWT inspection in integration tests, parse the JWT manually (split on '.', base64-decode the payload) — no extra library needed in Dart. Use a `FakeSecureStorageAdapter` (implements the interface, backed by a plain Map) as a test double rather than mocktail for the SecureStorage, to avoid verbose stubbing of every key.

For the restart simulation, do NOT call `dispose()` on the first instance — instead instantiate a second `TenantSessionStore` with the same FakeSecureStorageAdapter instance to simulate reading persisted data.

Unit tests (flutter_test + mocktail): mock both SecureStorageAdapter and the Supabase client. Test the full state machine: no session → persistSelection → restoreSelection → clearSelection → no session. Verify rollback by making the mock RPC throw and asserting SecureStorage delete was called. Test null-safe deserialization by passing malformed JSON and asserting a safe fallback (null return, not crash).

Integration tests: authenticate against a test Supabase project, call persistSelection() with a known org ID, decode the JWT using a JWT library and assert the claim value, then simulate restart by creating a new TenantSessionStore instance and calling restoreSelection(). Run integration tests only when RUN_INTEGRATION_TESTS=true. Target 100% branch coverage on TenantSessionStore logic.