test(mqtt): use real repositories in SubscribeTest and cover the session update - #8093

Problem

SubscribeTest registered ChannelRepository and SessionRepository as Mockito mocks:

@Mock private ChannelRepository channelRepository;
@Mock private SessionRepository sessionRepository;

A mock answers null for every lookup, so in Subscribe.subscribe():

String clientId = Singleton.INST.get(ChannelRepository.class).get(ctx.channel());   // null
MqttSession session = Singleton.INST.get(SessionRepository.class).get(clientId);    // null
if (Objects.nonNull(session)) { ... }                                              // never entered

the branch that stores the granted subscriptions on the session never ran. All three tests only
looked at the SUBACK, so that half of subscribe() had no coverage at all — and the mocks are also
why the null-key crash described in #8092 stayed invisible from this test class.

What this changes

• ChannelRepository and SessionRepository are real instances; the connected channel is registered
  in the channel repository, so subscribe() resolves a real client id.
• New test testSubscribeStoresSubscriptionsInSession asserts the subscription reaches the session.
• SubscribeRepository and TopicRepository stay mocked: subscribe() writes to the first, and the
  second is only observed through verify(topicRepository, never()).

Verification

mvn -pl shenyu-protocol/shenyu-protocol-mqtt -Dtest=SubscribeTest test
Tests run: 4, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

Full module: 147 tests, and the only errors are the three pre-existing
MqttTransportHandlerTest ones described below — the same three that a clean master produces
locally, no new failure.

Notes

• MqttTransportHandlerTest fails with Singleton.get(InflightRepository.class) returning null
  when it does not run after a class that registers every repository (MqttBootstrapServerTest).
  That is a pre-existing order dependency in a different test class; it is fixed in fix(mqtt): scope session state per instance and guard a missing client id #8092 together
  with the repository scoping, not here.
• This supersedes test: isolate MQTT repository fixtures #8089, which adds the same real instances but keeps the @Mock fields with the
  same names (variable channelRepository is already defined in class) and does not compile.