fix: use injected bookingEventHandler for proper DI

[hariombalhara]

What does this PR do?

This PR fixes CI test failures in the booking-audit-di branch by using the injected bookingEventHandler dependency instead of creating a new instance locally in RegularBookingService.

Context: This is a follow-up fix for PR #25123 which added DI infrastructure for the BookingAudit system. CI was failing with "Unhandled Errors" about emails not being sent in reschedule.test.ts and roundRobinManualReassignment.test.ts, even though tests passed locally.

Root cause: The code was creating a new BookingEventHandlerService instance instead of using the injected dependency from deps.bookingEventHandler. This prevented tests from properly stubbing/observing the handler, causing CI failures.

The fix: Changed line 2261 in RegularBookingService.ts to use deps.bookingEventHandler directly, aligning with the DI pattern introduced in the parent PR.

Visual Demo

N/A - This is an internal refactoring to fix test failures. No user-facing changes.

Mandatory Tasks

• I have self-reviewed the code
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - No documentation changes needed for this internal refactoring.
• I confirm automated tests are in place that prove my fix is effective or that my feature works. Existing unit tests pass locally (3921 tests)

How should this be tested?

Prerequisites:

• This PR targets the booking-audit-di branch, not main
• No special environment variables needed

Testing steps:

1. Checkout this branch
2. Run TZ=UTC yarn test to verify all unit tests pass
3. Wait for CI to complete and verify that the previously failing tests now pass:
   • Tests / Unit (reschedule.test.ts)
   • Tests / Unit (roundRobinManualReassignment.test.ts)

Expected behavior:

• All unit tests should pass both locally and in CI
• No "Unhandled Errors" about emails not being sent

Checklist

• I have read the contributing guide
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas N/A - Simple one-line change
• I have checked if my changes generate no new warnings

Important Notes for Reviewers

⚠️ Critical items to verify:

1. DI wiring completeness: Please verify that all call sites instantiating RegularBookingService provide the bookingEventHandler dependency. Check:

   • packages/features/bookings/di/RegularBookingService.module.ts
   • Any other places where RegularBookingService is constructed

2. CI validation: This fix is based on analysis of CI failures that didn't reproduce locally. The actual validation will happen when CI runs on this PR.

3. Base branch: This PR targets booking-audit-di, not main. It should be merged to booking-audit-di first, then that branch can be merged to main.

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Devin session: https://app.devin.ai/sessions/343c8e5dcda94d85b77a3eb65a30a7c6
Requested by: @hariombalhara (hariom@cal.com)

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