Await provider.send in sender worker

[Dobrunia]

Summary

Await all provider.send() calls so RabbitMQ messages are acked only after delivery finishes, and send failures can be retried/stashed.

[Copilot]

Pull request overview

This PR updates the sender worker to await all provider.send() calls so that task completion (and downstream RabbitMQ acking) only occurs after notification delivery has finished, allowing send failures to propagate and be retried by the worker mechanism.

Changes:

• Added await to previously fire-and-forget this.provider.send(...) calls in the sender worker task handlers.
• Added a regression test ensuring handle() rejects when provider.send() rejects.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
workers/sender/src/index.ts	Ensures task handlers await provider.send() so failures propagate and completion reflects actual delivery outcome.
workers/sender/tests/worker.test.ts	Adds coverage that handle() rejects if provider.send() fails (verifies the new awaiting behavior).

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