fix: handle PHP mocks and subclasses of Rust-backed classes

[ptondereau]

Description

Fixes panics when PHPUnit mocks or subclasses don't call the parent constructor.

• Add the default_init() trait method for types deriving Default
• Fall back to standard PHP handlers for uninitialized objects
• Add from_zend_obj_mut_uninit for constructor access
• Check the handler's pointer to reject non-native objects

[coveralls]

Pull Request Test Coverage Report for Build 21368712391

Details

• 8 of 64 (12.5%) changed or added relevant lines in 6 files are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 35.363%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/class.rs	0	2	0.0%
src/zend/ex.rs	0	2	0.0%
crates/macros/src/class.rs	8	12	66.67%
src/builders/class.rs	0	4	0.0%
src/types/class_object.rs	0	13	0.0%
src/zend/handlers.rs	0	31	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/types/class_object.rs	1	0.0%
src/zend/handlers.rs	7	0.0%

Totals
Change from base Build 21336425804:	-0.1%
Covered Lines:	1911
Relevant Lines:	5404

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💛 - Coveralls