fix(arrow/util): release protobuf enum dictionaries

[fallintoplace]

What changed

Release the temporary string builder and array used to initialize protobuf enum dictionaries.

Why

InsertStringDictValues copies values from its input. The temporary builder and array remained owned by the conversion path and leaked after each enum append.

Testing

• go test ./arrow/util
• Added checked-allocator coverage for enum conversion

[zeroshade]

LGTM — clean, minimal leak fix.

• getDictValues now defer bldr.Release()s the temp string builder; since the defer runs after bldr.NewArray() computes the return value, the returned array keeps the transferred buffers and the builder's own allocation is freed.
• The DICTIONARY path releases dictValues after InsertStringDictValues copies from it, and before the err check, so the error path doesn't leak either.
• The regression test is a real reproduction: with the fix it passes, and reverting protobuf_reflect.go makes the checked allocator report LEAK of 64 bytes for the temp builder/array.

Verified locally: go test ./arrow/util passes with no regressions. (The go vet note on protobuf_reflect.go:199 about copying a protobuf Any lock is pre-existing and unrelated to this change.)