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fix: leak one refcount in ZBox<ZendClassObject<T>>::set_zval (#735)

* fix: leak one refcount in ZBox<ZendClassObject<T>>::set_zval

ZBox::into_raw transfers ownership without touching the
refcount, so the box's single ref carries into set_object.
set_object then calls inc_count, bumping the count to 2.
The zval drops one ref on destruction; the object never
reaches zero and leaks.

On a PHP debug build the leak cascades into an assertion in
_zend_hash_str_add_or_update_i at module shutdown: leaked
instances keep shared class storage (used by classes with
#[php(prop)] fields) alive past its expected lifetime, and
property cleanup hits a frozen hashtable without the
HASH_FLAG_ALLOW_COW_VIOLATION bit.

Apply the same fix already present on the sibling impl
ZBox<ZendObject>::set_zval in object.rs: call dec_count
before into_raw so the net post-insertion count stays at 1.
The matching impl for RegisteredEnum + RegisteredClass in
enum_.rs had the same bug and is fixed alongside.

* test: regression coverage for ZendClassObject<T> set_zval refcount leak

Adds an integration test that exercises the exact buggy code path
fixed in the previous commit: a class extending \Exception with a
#[php(prop)] field, constructed via ZendClassObject::new(...)
.into_zval(...) and thrown via PhpException::with_object. The
caught exception's refcount is read inside the catch block via
debug_zval_dump; with the fix the only holder is the catch-bound
variable, so the dump reports refcount 2. Without the fix the
buggy set_zval keeps an extra ref and the dump reports 3.

Refcount via debug_zval_dump was chosen over a memory-growth
measurement so the same assertion fires on release and debug PHP
builds. memory_get_usage / gc_collect_cycles interact poorly with
a separate latent refcount bug in the IntoZval impl for
&mut ZendClassObject<T> (out of scope here): on debug builds GC
tries to add to a shared HT and trips
_zend_hash_str_add_or_update_i.

Verified against PHP 8.4.20 NTS ... (continued)

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