What does this PR do?

This PR fixes a critical scheduling bug where attendee availability was not being checked during reschedule operations, leading to double-booking scenarios. When a Cal.com user books another Cal.com user and the host tries to reschedule, the system was only checking the host’s availability and ignoring attendee conflicts.

• Fixes #16378
• Fixes CAL-4531

Bounty

• /claim #16378

The Problem:

• User A books User B for 2PM-3PM
• User B wants to reschedule to 3PM-4PM
• System only checks User B’s availability
• User A might be busy 3PM-4PM or not available on particular day ( Double-booking created! )

The Solution:

• Automatically detect Cal.com attendees during reschedule operations
• Include attendee availability in the core slot calculation system
• Block conflicting time slots from being offered
• Prevent double-booking scenarios entirely

Visual Demo (For contributors especially)

Before fix:

User A was unavailable on Wednesday, but User B (the host) was still able to reschedule to Wednesday.

https://github.com/user-attachments/assets/aa1f7078-9794-4996-a39f-aea975bc37fd

After fix:

User A was unavailable on Friday, so User B (the host) was not able to reschedule to Friday.

https://github.com/user-attachments/assets/50d77319-7065-40c3-8713-831cf52048f3

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. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. Create two Cal.com users:

• User A
• User B

2. Environment variables:

• No special environment variables required
• Standard Cal.com development setup

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
• I have checked if my changes generate no new warnings