Add missing convertArgs for primitive field setters in MethodHandle

[blogcin]

Fixes #12596

The REF_putField and REF_putStatic branches in
Util_java_lang_invoke_MethodHandle.invokeInternal(...) call
checkArgs(...) but skip convertArgs(args, methodType), so boxed
arguments are never unboxed before reaching the field accessor.
Every other invocation branch already calls convertArgs(...).

This patch adds the missing convertArgs(...) call in both branches
and includes a regression test (GitHub12596) that covers boolean,
byte, short, and char fields for both instance and static cases.

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