feat: check if guest is available when rescheduling

[retrogtx]

What does this PR do?

Does not allow the time slot to be booked in terms of rescheduling for event type owner if the booker is already busy

• Fixes #16378 (GitHub issue number)
• Fixes CAL-4531

https://www.loom.com/share/ece1aab754234614969cc8d24b44540c?sid=68d38af4-7c00-43dd-87db-afbe60ac8719

Mandatory Tasks (DO NOT REMOVE)

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

/claim #16378

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"Add consumer team as reviewer" took an action on this PR • (02/05/25)

1 reviewer was added to this PR based on Keith Williams's automation.

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1 reviewer was added to this PR based on Keith Williams's automation.

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