Stdlib: encoding/binary builtins reject null via typed string guards (#18252)

[PurHur]

Summary

• Switch bin2hex, base64_encode, quoted_printable_encode, and quoted_printable_decode VM paths from VmString::coerceStringBuiltinArg to coerceTypedStringBuiltinArg so null operands throw TypeError like Zend PHP 8.2+.
• Align JIT lowering via JitStringBuiltinArg::lowerTypedString (same pattern as closed Regression: typed string builtins — null $string coerced to '' not TypeError (ext/standard/string.c, ext/standard/url.c) #18190 urlencode/htmlspecialchars).
• No C runtime changes — PHP SSOT in ext/standard/ + shared JitStringBuiltinArg guards.

Closes #18252

php-src reference

• ext/standard/string.c — php_bin2hex, php_quoted_printable_encode/decode
• ext/standard/base64.c — php_base64_encode

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php test/repro/maintainer_gap_encoding_binary_null.php'

bin2hex(): Argument #1 ($string) must be of type string, null given
base64_encode(): Argument #1 ($string) must be of type string, null given
quoted_printable_encode(): Argument #1 ($string) must be of type string, null given
quoted_printable_decode(): Argument #1 ($string) must be of type string, null given

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/jit.php test/repro/maintainer_gap_encoding_binary_null.php'

Same output (JIT agrees).

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter encoding_null'

OK (4 tests, 4 assertions)

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