[#209] Add per-tenant snapshotting to the multi-tenancy demo

[laura-devriendt-lemon]

Adds per-tenant snapshotting to the multi-tenancy demo, the half of axoniq-framework#263 the demo did not yet cover. Tracked by axoniq-framework#209, which is what the #209 in the title refers to rather than anything in this repository.

What it adds

A new step 2 in DemoLifecycle.run. Both known tenants fill a course under the same identifier, so each tenant's own snapshot store ends up holding its own snapshot of that identifier. The run logs:

Course [cs-101] is snapshotted in both tenants' own snapshot stores: true.
Each snapshot holds only its own tenant's student, so neither read the other's: true.
Springfield's snapshot holds [alice], Shelbyville's holds [carol]

Snapshot isolation cannot be shown behaviourally, because a snapshot changes no behaviour. The demo therefore reads the two per-tenant stores through a new TenantSnapshots seam and compares snapshot contents. Comparing the framework's snapshot envelopes would prove nothing, since each envelope carries its own write timestamp and never compares equal to another, even if one tenant had read the other's snapshot.

Also included

MultiTenancyDemoIT failed about one run in four with CANCELLED: Unknown Context springfield on its first command. Provisioning waits until the tenant provider lists a tenant, which does not mean Axon Server routes to its context yet. The existing Enrollments.causedByTenantNotReady retry now covers every tenant's first command, not only the runtime-added one.

Depends on axoniq-framework#263

TenantSnapshotStoreFactory exists only on that branch, so examples CI fails here until it merges. Verified locally against feature/176/209_eventStore at cfa9839b92, where the 12 core tests, the declarative smoke test, and the Axon Server integration test all pass.

[smcvb]

No concerns here. But, I must say, this "example" looks more like an extremely extensive integration test than a user-facing example. In other words, it's an example for us to ensure multi-tenancy works as intended, not an example to the user how to enable and use multi-tenancy.

Letting it slide, as I don't think we should enforce that distinction right now, but it's worth noting that the examples folder is essentially intended as material for users to comprehend how to achieve something. Having things like demo life cycle and outcome objects, to me, don't belong in a user's application, those belong in test functionality.

FYI, the examples build fails, so we should fix that before merging :-)