Fix ?? on property hooks without get-hook invocation (#8902)

[PurHur]

Summary

• ?? / ??= on hooked properties now null-check and read backing storage instead of invoking get hooks (parity with Zend/zend_property_hooks.c, pairs with Language: ??= on property hooks must invoke get/set hooks (zend_property_hooks.c) #6472 ??=).
• Compiler: property-fetch coalesce lowering (issetForCoalesceAssign, propertyHookCoalesceRead), stmt-level PropertyFetch; Coalesce fusion, and call-arg temp clone resolution (Regression: backed enum case loose == with backing scalar returns true (re-#5798, zend_operators.c) #8766).
• VM: fetchObjectPropertyForCoalesce() for left-branch reads without get hook.

php-src reference

• Zend/zend_property_hooks.c — isset/coalesce backing probe
• Zend/zend_compile.c — ZEND_AST_COALESCE on hooked properties

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro/issue_property_hook_coalesce.php
./vendor/bin/phpunit --filter property_hook_coalesce
./vendor/bin/phpunit --filter property_hook_nullcoalesce_assign
'

Output:

string(1) "a"
ok
string(7) "default"
unset ok
OK (1 test, 1 assertion)  # property_hook_coalesce
OK (1 test, 1 assertion)  # property_hook_nullcoalesce_assign

Closes #8902

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