add deterministic notification pipeline smoke test

[canicefavour]

Implemented a fast, deterministic smoke test covering the notification lifecycle from local event ingestion through notification payload generation, without relying on external notification providers.

Implementation
Added a dedicated smoke test covering the local notification ingestion and processing pipeline.
Exercised event validation, routing, notification template resolution, and internal dispatching.
Added mocked/in-memory delivery handling to isolate the pipeline from external push, email, SMS, and webhook providers.
Verified representative event payloads are processed successfully through the notification workflow.
Asserted generated notification payloads for the expected recipient, title, body, and metadata.
Verified the mocked transport receives the generated notification exactly once.
Confirmed no real outbound network requests are made during the test.
Added bounded asynchronous handling and clean teardown to prevent hanging test processes.
Ensured the smoke test runs without external credentials, databases, or third-party API dependencies.
Added a dedicated command for executing the smoke test locally and in CI.
Documented the execution requirements and offline CI assumptions.
Verified the test completes quickly and deterministically with clean process termination.
Result

The notification system now has an isolated end-to-end smoke check that provides rapid CI feedback on event processing and notification generation while remaining fully offline and independent of external delivery services.

Closes #723

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