Race in MockProvider mock-message access across overlapping Subscribe goroutines

[miotte]

The package-level mockp is shared across tests in internal/server. MockProvider.Subscribe runs in a goroutine that defers a 500ms sleep before returning, so that goroutine can outlive the test that triggered it. Its read/reset of MockMessages then races against the next test's Subscribe goroutine when -race is enabled.

The slice should be encapsulated behind synchronization (e.g. a sync.Mutex plus SetMockMessages / takeMockMessages helpers) so concurrent tests can't observe or mutate it without coordination.