Create reusable utility for agenda folder validation

[coderabbitai[bot]]

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Background

Currently, agenda folder validation logic is duplicated across different mutations. This includes checking if:

• The folder exists
• The folder belongs to the correct event
• The folder is compatible with agenda items

Proposal

Create a reusable utility function to centralize this validation logic, making the code more maintainable and reducing duplication.

References

• Discussion in PR #3216

Notes

• This improvement was initially considered during PR #3216 but was deferred to maintain PR focus
• Implementation details will be discussed in the PR that addresses this issue

/cc @shivasankaran18