test: add Atomizer full flow tests

[mnbuhl]

Summary

• Adds a dedicated Atomizer.FlowTests project for hosted end-to-end flow coverage across InMemory, Redis, PostgreSQL, MySQL, and SQL Server storage drivers.
• Covers P0, P1, and P2 flows including enqueue/process, scheduling, recurring jobs, ExecuteAsync, retries/failures, idempotency, partitions, queue routing, misfires, heartbeat recovery, shutdown release, and retention cleanup.
• Keeps repeated setup in shared host, driver, recorder, timing, and test job helpers.

Changes

• Registers Atomizer.FlowTests in the solution with Testcontainers-backed EF/Redis fixtures and xUnit collection isolation.
• Adds shared flow-test infrastructure for hosted Atomizer processing, storage reset, polling/wait helpers, and deterministic test timings.
• Exposes queue tick interval internally for test assemblies and grants InternalsVisibleTo for the flow test project.

[github-actions]

Package	Line Rate	Health
Atomizer	78%	✔
Atomizer.Redis	87%	✔
Atomizer.EntityFrameworkCore	92%	✔
Atomizer	76%	✔
Atomizer.Redis	88%	✔
Atomizer	27%	❌
Atomizer.EntityFrameworkCore	93%	✔
Atomizer	27%	❌
Atomizer	78%	✔
Atomizer.EntityFrameworkCore	93%	✔
Atomizer	27%	❌
Atomizer.EntityFrameworkCore	93%	✔
Atomizer	27%	❌
Atomizer	78%	✔
Atomizer.Redis	88%	✔
Atomizer	27%	❌
Summary	63% (16189 / 26249)	➖