fix: Restore member xmlName resolution in XML builder root entry pt

[jterapin]

Summary

The build entry point only checks shape.target.traits['smithy.api#xmlName'] for the root element name, but doesn't check shape.traits['smithy.api#xmlName'] (the member-level xmlName). This was a regression introduced in #342 — the old code checked ref.location_name first, which callers used to pass the resolved member xmlName.

This matters for REST XML httpPayload members where the member's xmlName overrides the target shape's xmlName (e.g., member has xmlName: "Hola", target has xmlName: "Hello" — should serialize as <Hola>).

Changes

• Check shape.traits['smithy.api#xmlName'] before shape.target.traits['smithy.api#xmlName'] in the builder's root resolution — consistent with how nested members are already resolved via the location_name helper.

Test case

HttpPayloadWithMemberXmlName protocol test — member xmlName should win over target xmlName for the root element. All protocol tests passes.