php-in-php: drop UserScriptAotDeferNestedJit from StringBin2hex (#20011)

[PurHur]

Summary

• Remove UserScriptAotDeferNestedJit from StringBin2hex; thin hex via Context::isThinStandaloneAotMain() + existing JitBin2hexKernel.
• Embed / non-thin path still JitVmHelperLink::ensureBridge → Bin2hexJitHelper PHP.
• Pre-link bin2hex in ensureMinimalUserStandaloneBodies (same as readfile).

C / defer surface

• Deleted: UserScriptAotDeferNestedJit import + shouldDefer() gate from StringBin2hex (−1 defer call site).
• Kept: thin JitBin2hexKernel hex loop (not libc; no new C).

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit test/unit/Bin2hexRuntimeShrinkTest.php
vendor/bin/phpunit test/aot/AotTest.php --filter "bin2hex\.phpt"
php script/bootstrap-inventory.php --check
php script/check-selfhost-spine-coverage-sync.php
make bootstrap-selfhost-vm-driver-execute-probe
'

• Shrink: OK (3 tests, 26 assertions)
• AOT bin2hex.phpt: OK
• inventory: OK 5347/5347; spine sync OK; vm-driver-probe OK
• Note: bin2hex_hex2bin_enum_typeerror AOT still fails on master via StringHex2bin nested JIT (Object_::compileStaticPropertyIsSet TypeError) — pre-existing, not this PR.

Closes #20011

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