Unify scheduled message store via PersistAsync

[PascalSenn]

Summary

Collapses the two scheduling code paths in Mocha (middleware-store vs. endpoint-native) into a single resolver-routed IScheduledMessageStore.PersistAsync call. Removes SchedulingTransportFeature.SupportsSchedulingNatively.

• IScheduledMessageStore.PersistAsync now takes IDispatchContext. The store reads envelope, scheduled time, transport, and endpoint from the context.
• New IScheduledMessageStoreResolver selects the store for a scheduled dispatch by transport type and for cancellation by token prefix.
• DispatchSchedulingMiddleware is always present in the default dispatch pipeline; resolves and invokes the right store at runtime.
• Azure Service Bus and Postgres transports each register a transport-specific store; EfCoreScheduledMessageStore becomes the optional fallback for non-native transports.
• AzureServiceBusDispatchEndpoint and PostgresDispatchEndpoint lose their ScheduledTime branches and become send-now terminals. Provisioning + entity-path resolution + message construction are extracted to shared helpers.
• Scheduled dispatch to an unsupported transport now throws NotSupportedException instead of being sent immediately.

The full design rationale, alternatives considered, and trade-offs are in plan.md.

Test plan

• dotnet build src/Mocha/Mocha.slnx — 0 errors
• dotnet test src/Mocha/test/Mocha.Tests --filter "FullyQualifiedName~Scheduling|FullyQualifiedName~ScheduledMessage" — 45/45 pass on net8.0/net9.0/net10.0
• Mocha.Transport.InMemory.Tests — 162/162 pass (covers redelivery + scheduling regressions)
• Mocha.Sagas.Tests — 127/127 pass (saga timeouts via ScheduleSendAsync)
• Resolver routing + registration validation unit tests
• Squadron-fixture tests against real ASB broker — Mocha.Transport.AzureServiceBus.Tests/Behaviors/SchedulingTests.cs
• Squadron-fixture tests against real Postgres — Mocha.Transport.Postgres.Tests/Behaviors/SchedulingTests.cs
• RabbitMQ fixture tests (rewritten to assert the new throw contract)
• EF Postgres integration tests
• Session-aware ASB scheduled-dispatch integration test (locks the pipeline-order invariant)