Fix session_start AOT abort + false bool return (#21892)

[PurHur]

Summary

• Closes Regression: session_start() AOT aborts in SessionCreateIdJitHelper::randomIdString — nested VmSession::createId (#1974) #21892 (progress CI: Default SESSIONS_WEB_DEPLOY_SMOKE_GATE=1 in ci-local after #1967 green #1974): stop nested-JIT VmSession::createId() from SessionCreateIdJitHelper — inline php-src bin_to_readable / random_bytes (sid_length=26).
• JitValueBox bool copy: __value__readLong has no TYPE_NATIVE_BOOL arm and returned 0, so $r = session_start() was always false on AOT after a successful start. Read the bool payload byte instead.
• Already-active session_start LLVM path returns true (php-src); emitWriteBool uses __value__writeBool.
• Session setcookie bridge passes empty strings (not null __string__*) for domain/samesite.

Still blocked for #1974 deploy smoke

SessionsWebAotExecuteTest still fails: thin AOT keeps __phpc_setcookie_add as a no-op link stub. Upgrading stubs to NestedJIT PendingHeadersJitHelper currently segfaults in addHeader when invoked from session (follow-up issue).

Test plan

• AOT: $r=session_start(); echo $r?"T":"F" → T (was abort / F)
• VM same → T
• SessionCreateIdRuntimeShrinkTest + session shrink tests
• SessionsWebAotExecuteTest — still fails (Set-Cookie stub); tracked for follow-up

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
printf "%s\n" "<?php" "\$r=session_start();" "echo \$r?\"T\":\"F\";" "echo \"\\n\";" > /tmp/sst.php
PHP_COMPILER_LLVM_ASSERT=1 php bin/compile.php -o /tmp/sst.bin /tmp/sst.php
/tmp/sst.bin
php -d memory_limit=1536M vendor/bin/phpunit --filter SessionCreateIdRuntimeShrinkTest
'

php-src: ext/session/session.c (create id / start). PHP-in-PHP: ext/standard/SessionCreateIdJitHelper.php, lib/JIT/JitValueBox.php.

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