Fix optional/capi/ext/digest_spec.c on Ruby 3.4.2

Original error:

```
$ ruby /Users/runner/work/spec/mspec/bin/mspec-run :capi
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [arm64-darwin23]
digest_spec.cpp:106:12: error: assigning to 'rb_digest_metadata_t *' from incompatible type 'void *'
    algo = RTYPEDDATA_DATA(obj);
           ^~~~~~~~~~~~~~~~~~~~
/Users/runner/hostedtoolcache/Ruby/3.4.2/arm64/include/ruby-3.4.0/ruby/internal/core/rtypeddata.h:102:38: note: expanded from macro 'RTYPEDDATA_DATA'
#define RTYPEDDATA_DATA(v)           (RTYPEDDATA(v)->data)
                                     ^~~~~~~~~~~~~~~~~~~~~
1 error generated.
make: *** [digest_spec.o] Error 1
```

Author: andrykonchin

optional/capi/ext/digest_spec.c

@@ -103,7 +103,7 @@ static rb_digest_metadata_t * get_metadata_ptr(VALUE obj) {
     // In the digest gem there is an additional data type check performed before reading the value out.
     // Since the type definition isn't public, we can't use it as part of a type check here so we omit it.
     // This is safe to do because this code is intended to only load digest plugins written as part of this test suite.
-    algo = RTYPEDDATA_DATA(obj);
+    algo = (rb_digest_metadata_t *) RTYPEDDATA_DATA(obj);
 #else
 # undef RUBY_UNTYPED_DATA_WARNING
 # define RUBY_UNTYPED_DATA_WARNING 0