refactor: move XML shape metadata into smithy-xml extensions

[jterapin]

Context

This is the third PR in the stack and should be reviewed on top of #353.

PR #353 moved JSON and CBOR wire-name resolution into protocol-specific extensions and normalized generated runtime metadata around model_name and symbolized serde traits. This follow-up does the same cleanup for XML: move XML-specific metadata and lookup logic into Smithy::Xml::Extension while keeping restXml behavior stable.

What changed

• Added Smithy::Xml::Extension helpers for XML serde metadata derived from Smithy traits.
• Moved XML structure/member naming through extension helpers that prefer the Smithy @xmlName trait.
• Moved XML attribute vs element partitioning through extension helpers that read the Smithy @xmlAttribute trait.
• Moved XML namespace attribute derivation through extension helpers that read the Smithy @xmlNamespace trait.
• Updated the XML parser to resolve modeled members through an XML member index built by the extension.
• Updated XML-related codegen expectations so the runtime path continues to use model_name and symbolized serde traits consistently.

Non-goals

• No JSON or CBOR behavior changes in this PR.
• No Query runtime compatibility change in this PR.

Testing

• bundle exec rspec gems/smithy-xml/spec/smithy-xml/extension_spec.rb gems/smithy-xml/spec/smithy-xml/builder_spec.rb
• bundle exec rspec gems/smithy/spec/interfaces/schema/serde_traits_spec.rb
• bundle exec rake test:protocols:restXml in aws-sdk-ruby-staging
  • result: 256 examples, 0 failures, 2 pending

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Written with AI assistance and reviewed by jterapin.