Improve client hook subscriber management

[Lms24]

Problem Statement

Today, we simply store client hook callbacks in an array per hook. This generally works fine, but as we learned in #17272, causes incorrect behaviour when removing a callback (i.e. unsubscribing) synchronously while subscribers are still being invoked.

Solution Brainstorm

Ideally, we can find a solution that

• enables synchronous unsibscribing without compromising other subscribers
• requires no breaking API changes

[linear]

JS-813 Improve client hook subscriber management