refactor: use calendar event builder

[Udit-takkar]

What does this PR do?

Mandatory Tasks (DO NOT REMOVE)

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

How should this be tested?

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Summary by cubic

Refactored RegularBookingService to use CalendarEventBuilder for consistent event construction and updates. Adds new builder methods to reduce direct mutations and simplify reschedule/create flows.

• Refactors

  • Replaced direct evt mutations with builder calls in reschedule and create paths.
  • Used builder to set attendeeSeatId, appsStatus, video call data, iCalUID, and description.
  • On organizer change, cleared video data, removed iCalUID, and marked organizer changed via builder.

• New Features

  • CalendarEventBuilder: added withTitle, withDescription, withRescheduledBy, and withOrganizerChanged.

^{Written for commit 3639b38. Summary will update on new commits.}