php-in-php: route session_encode/decode JIT through SessionEncodeJitHelper (#9440)

[PurHur]

Summary

• Replace LLVM wire serializer loops in SessionEncodeRuntime.php (~530→304 lines) with thin bridges into SessionEncodeJitHelper → VmSessionSerializer PHP
• Add SerializeJitHelper for JIT session wire value serialization without VM Context
• VM decodePhp / encodePhp unchanged (php-src-strict); JIT wire path uses VmUnserializeFormat + SerializeJitHelper

Closes #9440

php-src reference

• ext/session/session.c — php_session_encode() / php_session_decode()
• ext/session/mod_php.c — php handler wire format

LLVM shrink

• Removed: ~226 lines from SessionEncodeRuntime.php (emitEncodeWire, emitDecodeWire, emitHashtableSetParsed, libc memchr unserialize loop)
• Added: thin bridges + SessionEncodeJitHelper.php / SerializeJitHelper.php

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter SessionEncode test/unit/SessionEncodeRuntimeShrinkTest.php'
# OK (2 tests, 12 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter session_encode test/compliance/'
# OK (6 tests, 6 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php -r '"'"'session_start(); $_SESSION["k"]="v"; echo session_encode();'"'"'
php bin/jit.php -r '"'"'session_start(); $_SESSION["k"]="v"; echo session_encode();'"'"''
# Both print: k|s:1:"v";

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