Test listSpreadRhs guard without php-cfg overlay (#5472)

[PurHur]

Summary

• Closes Bootstrap: php-cfg list-spread overlay missing — Undefined property Assign::$listSpreadRhs warnings (zend_compile.c) #5472 — assignIsListSpread() in lib/Compiler.php already guards all listSpreadRhs reads with property_exists() (landed via Language: enum case instanceof user interface — regression vs #3373 (zend_enum.c) #5703/php-in-php: listSpreadRhs php-cfg patch — eliminate compile-time warnings on list()/closure scripts #6069)
• Adds regression test that temporarily strips listSpreadRhs from vendor Assign.php and verifies $a = 1; compiles without warnings

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php -r "\$a=1;" 2>&1 | grep listSpreadRhs && exit 1 || exit 0'
# exit 0 — no warnings

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter ListSpreadRhsCompileTest'
# OK (4 tests, 8 assertions)

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