[php-src] PR #24751: Fix GH-24674: Share the unserialize context only with Serializable

Pull Request: https://github.com/php/php-src/pull/24751 Author: iliaal php_var_unserialize_init() hands the context of a parse in progress to any caller, so user code that runs mid-parse and calls unserialize() (a destructor from a failed __unserialize() payload or the GC, an error handler, an allowed_classes __toString(), a session_decode() overwrite) resolves R: and r: into the outer value, including values being destroyed. The context is now shared only with the unserialize handler of a C: payload, and only while serialize_lock is not held, as serialize() does; fiber switches inside that handler now throw FiberError, so this targets master. #24433 fixes the reported __unserialize() case on PHP-8.4. Fixes #24674