Chore: Migrate away from mobx decorators to avoid runtime implications in typescript configuration

[AtofStryker]

What would you like?

Cypress would like to remove the use of mobx decorators as they are now opt-in in mobx version 6

Why is this needed?

To avoid heavy runtime implications of useDefineForClassFields, which is needed for mobx decorators. This caused issues when #30764 was introduced which removed the field and caused a failure we did not catch until the Cypress 14 bughunt. #30856 reverted the change. We would like to avoid as much runtime manipulation as possible, which is the rationale for migrating away from decorator patterns.

Original mobx update PR can be found in #30514

Other

No response