test: add event bus listener removal mid-emit tests (Issue #131)

[rafaio1]

Summary

Adds tests verifying RuntimeEventBus behavior when a listener unsubscribes another listener during emission.

Tests Added

• ✅ Unsubscribing another listener mid-emit does not break iteration
• ✅ Removed listener never fires on subsequent emissions
• ✅ Remaining listeners continue to fire after mid-emit unsubscribe

Verification

All 3 tests pass locally via npx vitest run tests/events/mid-emit-unsubscribe.test.ts.

Closes #131