Harden BackedEnum from()/tryFrom() lookup (#9603)

[PurHur]

Summary

• Add constants-table fallback in BackedEnum::caseForValue() when ClassEntry::$enumCases is empty or stale (Zend zend_enum_from_case parity).
• Resolve live runtime enum via EnumSupport::resolveRuntimeEnumClass() in EnumFromHandler::execute().
• Add issue Language: Enum::tryFrom()/from() — valid backing values return NULL / ValueError (Zend/zend_enum.c) #9603 repro script and PHPUnit guards (VmEnumFromTest, VmEnumCanonicalCaseVariableTest).

Related: #9575 fixed the primary regression on master; this hardens edge cases where case lookup could still miss valid backing values.

php-src reference

• Zend/zend_enum.c — zend_enum_from(), backed-value case hash

PHP implementation

• lib/VM/BackedEnum.php — constants fallback lookup
• lib/VM/EnumFromHandler.php — runtime enum resolution

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit test/unit/VmEnumFromTest.php --filter Issue9603
vendor/bin/phpunit test/unit/VmEnumCanonicalCaseVariableTest.php --filter testFromResolvesWhenEnumCasesTableEmpty
vendor/bin/phpunit test/compliance/EnumVMTest.php --filter "enum_try_from|enum_from"
php bin/vm.php test/repro/issue_9603_enum_from.php
php bin/vm.php test/repro/maintainer_enum_from_regression.php
'

Output:

OK (1 test, 1 assertion)   # Issue9603
OK (1 test, 2 assertions)  # empty enumCases fallback
OK (6 tests, 6 assertions)  # compliance enum_from/tryFrom
NULL / enum(E::A) / from ok  # issue repro
enum(Color::Red) ... (maintainer repro green)

Closes #9603

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