Add destination assertions to rule tests

[tomasz-sq]

Why

Rule YAML tests can currently observe destination routing but cannot assert whether a matching event should route an alert to a specific destination or remain signal-only (destination list is empty)

What

• Add optional ExpectedDestinations lists to rule tests
• Treat an empty expected list as signal-only routing
• Compare named expectations against resolved test destinations
• Add passing and mismatch regression coverage

Risk Assessment

Low. Existing tests omit the optional field and retain their current behavior.

[cursor]

PR Summary

Low Risk
The feature is entirely optional and backward compatible; tests without ExpectedDestinations retain their current behavior unchanged.

Overview
This PR adds the ability for rule tests to assert expected alert destination routing by introducing an optional ExpectedDestinations field to test specifications.

Tests can now specify which destinations an alert should route to, with an empty list representing signal-only (no destination routing). The implementation creates test-specific destination instances for any expected destinations not available globally, then compares the actual routing output against the expected list to mark tests as failed if they don't match.

Includes schema validation updates, test utilities, and regression test fixtures demonstrating the feature including a passing test case and a mismatch failure case.

^{Reviewed by Cursor Bugbot for commit 8e1140f. Bugbot is set up for automated code reviews on this repo. Configure here.}

[dekatzenel]

Other than the issue cursor flagged, LGTM. I'll wait on that fix to approve. Tag me when it's ready!

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

^{Reviewed by Cursor Bugbot for commit 269e893. Configure here.}