refactor: use schema extensions for protocol-aware JSON/CBOR naming

[jterapin]

Context

This is the second PR in the stack and should be reviewed on top of #351.

PR #351 introduced a temporary reverse lookup on StructureShape so JSON and CBOR structure serde could operate on the actual payload or input data. That lookup is protocol-blind: a jsonName wire key can resolve even when the active protocol should be using the modeled member name instead.

This PR replaces that stopgap with lazy, protocol-aware naming resolution for JSON and CBOR.

What changed

• Added Smithy::Schema::Extension for lazy member indexes keyed by modeled member names.
• Added Smithy::Json::Extension for jsonName-aware lookup used only when JSON serde opts into json_name behavior.
• MemberShape now uses model_name as the primary modeled member identity.
• Codegen now emits model_name: and symbolized serde trait keys needed by the new lookup path.
• Removed the eager schema-wide reverse-map approach from the previous stopgap.

Behavior after this change

• JSON with json_name: true uses jsonName
• JSON with json_name: false uses the modeled member name
• CBOR uses the modeled member name
• Union __type is only preserved when it is explicitly modeled through jsonName

Explicit non-goals

• No XML behavior changes in this PR
• No Query behavior changes in this PR

Compatibility note

MemberShape#initialize still falls back from model_name: to location_name: for downstream compatibility. That fallback is currently needed because some downstream Query/XML wrapper-member paths still construct members with location_name:.

Testing

• bundle exec rake smithy-schema:spec
• bundle exec rake smithy-json:spec
• bundle exec rake smithy-cbor:spec
• bundle exec rake smithy:spec:unit
• bundle exec rake smithy:spec:protocols smithy:rbs:protocols
• bundle exec rake smithy-schema:rbs

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Written with AI assistance and reviewed by jterapin.