Fix trait codegen for structure's string member with @idRef

[joewyz]

Background

String shapes with @idRef are converted into ShapeId objects during the trait codegen. However, the generated fromNode() method and Builder setters for string members with @idRef within structures are incorrect.
Because the @idReft info is binding with the member shape and those incorrect methods are generated based on member shape's targets.

For example the following smithy model:

$version: "2.0"

namespace test.smithy.traitcodegen.structures

@trait
structure StructWithIdrefMember {
    @idRef(selector: "structure")
    idRefMemberA: String
    idRefMemberB: IdRefMemberB
}

@idRef(selector: "structure")
string IdRefMemberB

The generated fromNode() and builder's setter methods are incorrect.

@SmithyGenerated
public final class StructWithIdrefMemberTrait extends AbstractTrait implements ToSmithyBuilder<StructWithIdrefMemberTrait> {
    public static final ShapeId ID = ShapeId.from("example.traits#StructWithIdrefMember");

    private final ShapeId idRefMemberA;
    private final ShapeId idRefMemberB;
    /**
     * Creates a {@link StructWithIdrefMemberTrait} from a {@link Node}.
     *
     * @param node Node to create the StructWithIdrefMemberTrait from.
     * @return Returns the created StructWithIdrefMemberTrait.
     * @throws ExpectationNotMetException if the given Node is invalid.
     */
    public static StructWithIdrefMemberTrait fromNode(Node node) {
        Builder builder = builder();
        node.expectObjectNode()
            .getStringMember("idRefMemberA", builder::idRefMemberA)
            .getMember("idRefMemberB", n -> ShapeId.fromNode(n), builder::idRefMemberB);
        return builder.build();
    }
    
    public static final class Builder extends AbstractTraitBuilder<StructWithIdrefMemberTrait, Builder> {
        private ShapeId idRefMemberA;
        private ShapeId idRefMemberB;

        private Builder() {}

        public Builder idRefMemberA(String idRefMemberA) {
            this.idRefMemberA = idRefMemberA;
            return this;
        }
    }
}

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