Fix GH-20503: Assertion failure with ext/date DateInterval property hash construction

[heathdutton]

When a DateInterval object has a circular reference (e.g., $obj->prop = $obj), calling json_encode() triggers an assertion failure in debug builds. This happens because the get_properties handler modifies the properties HashTable in place, but circular references cause the HashTable to have a refcount > 1, which violates the assertion in zend_hash_str_update().

The fix adds a get_properties_for handler for DateInterval that returns a duplicated HashTable. To ensure circular reference detection still works (which relies on HashTable identity), the duplicated HashTable is cached in module globals, keyed by object handle. When we detect a recursive call (cache entry exists with refcount > 1), we return the same cached HashTable so that GC_IS_RECURSIVE can properly detect the cycle.

This approach avoids any ABI break by using module globals instead of adding a field to php_interval_obj.

Fixes GH-20503

[arnaud-lb]

Thank you @heathdutton! The approach looks right, but please check comments. (Edit: Please change the base of the PR to PHP-8.3. The branch is based on 8.3, but not the PR.)

[arnaud-lb]

NB: In master, DateInterval could be greatly simplified by using standard object properties and removing all the prop handlers.