fix: improve BookingReferenceRepository integration test isolation

[keithwillcode]

What does this PR do?

Fixes flaky integration tests in BookingReferenceRepository.integration-test.ts that were introduced by commit eecf9e1. The tests were failing with ForeignKeyConstraintViolation errors (Prisma error P2003) on the BookingReference_bookingId_fkey constraint.

Root cause: The tests created a single booking in beforeAll that was shared across all tests. This caused race conditions when tests run in parallel - one test's cleanup could delete the booking while another test was still trying to use it.

Fix:

• Use seed user (member0-acme@example.com) when available in CI, with fallback to creating a test user for local development
• Move booking creation from beforeAll to beforeEach so each test gets a fresh, isolated booking
• Track created bookings and clean them up properly in afterEach
• Fix import organization to satisfy linter

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - test-only changes.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. CI should pass - the integration tests that were previously flaky should now pass consistently
2. Run the integration tests locally multiple times to verify stability:

   set -a && source .env && set +a
   VITEST_MODE=integration TZ=UTC yarn test packages/features/bookingReference/repositories/BookingReferenceRepository.integration-test.ts

3. All 9 tests in the file should pass consistently across multiple runs

Checklist

• I have read the contributing guide
• My code follows the style guidelines of this project
• I have checked if my changes generate no new warnings

---

Reviewer notes:

• Verify the import change from import prisma from "@calcom/prisma" to import { prisma } from "@calcom/prisma" is correct
• The seed user fallback creates a test user that isn't explicitly cleaned up if the seed user doesn't exist - this is acceptable since it only happens in local dev

Requested by: @keithwillcode
Link to Devin run: https://app.devin.ai/sessions/a4fb71e0632041b28f8ed19205b96cf9

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