Improve tests for SyncService

Follow up https://github.com/nextcloud/mail/pull/12548

https://github.com/nextcloud/mail/blob/65a26b2441c025d916f91c44d39fe3219c688753/tests/Unit/Service/Sync/SyncServiceTest.php#L105

The sync() expectation is matching against fresh stubs created inline for the IMAP client and logger. SyncService is very likely to pass the exact instances it uses internally (client from the factory, logger from the constructor), so these should be captured into variables/properties and reused in with(...) (or replaced with looser constraints) to avoid brittle, failing argument matching.

https://github.com/nextcloud/mail/blob/65a26b2441c025d916f91c44d39fe3219c688753/tests/Unit/Service/Sync/SyncServiceTest.php#L114

syncStats() is asserted with a newly created IMAP client stub. This will not be the same instance as the client returned by the factory in this test, so the expectation is likely to fail. Reuse the same $client stub (or use a type/constraint matcher) for both the factory return and the syncStats() expectation.

https://github.com/nextcloud/mail/blob/65a26b2441c025d916f91c44d39fe3219c688753/tests/Unit/Service/Sync/SyncServiceTest.php#L96

In this test you create a new IMAP client stub inline in the willReturn(...) call, but later the expectations use different stubs for the IMAP client argument. If SyncService::syncMailbox() passes the client returned by IMAPClientFactory::getClient() through, these argument comparisons will not match and the test will fail. Assign the client stub to a variable (or use a constraint like isInstanceOf) and reuse it consistently across all expectations.

Open tests/Unit/Service/Sync/SyncServiceTest.php around lines 96, 105, and 114, then inspect how the IMAP client factory and SyncService are set up. Reuse the same client and logger stubs across the expectations, or apply the stated constraints, and run the SyncService unit test to confirm the argument matching works.