Remove unnecessary structs for BSON MinKey and MaxKey classes

[alcaeus]

Missed these empty structs when cleaning up structs and class declarations in #1992.

[Copilot]

Pull request overview

This PR continues the cleanup from #1992 by removing now-unnecessary internal structs and per-class object handlers for the BSON MinKey and MaxKey classes, relying on default Zend object allocation instead.

Changes:

• Remove the empty phongo_maxkey_t and phongo_minkey_t struct typedefs.
• Switch MAXKEY/MINKEY class declarations from PHONGO_DECLARE_CLASS_WITH_HELPERS to PHONGO_DECLARE_CLASS.
• Drop custom create_object/handlers boilerplate in src/BSON/MinKey.c and src/BSON/MaxKey.c.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/phongo_structs.h	Removes empty struct typedefs for MinKey/MaxKey.
src/phongo_classes.h	Updates class declarations to no longer require struct-based helper macros.
src/BSON/MinKey.c	Removes custom object handlers and create_object hook; keeps only class registration.
src/BSON/MaxKey.c	Removes custom object handlers and create_object hook; keeps only class registration.

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[GromNaN]

Less code is a great feature!