Add missing readonly traits on HTTP GET tests

[srchase]

Adds missing @readonly traits on HTTP GET protocol tests, eliminating the following warnings:

WARNING: aws.protocoltests.restjson#ResponseCodeRequired (HttpMethodSemantics)
   @ ~/smithy/smithy-aws-protocol-tests/model/restJson1/http-response-code.smithy
   |
 22 | @http(method: "GET", uri: "/responseCodeRequired", code: 200)
   | ^
   = This operation uses the `GET` method in the `http` trait, but is not marked with the readonly trait
WARNING: aws.protocoltests.restjson#ResponseCodeHttpFallback (HttpMethodSemantics)
   @ ~/smithy/smithy-aws-protocol-tests/model/restJson1/http-response-code.smithy
   |
 34 | @http(method: "GET", uri: "/responseCodeHttpFallback", code: 201)
   | ^
   = This operation uses the `GET` method in the `http` trait, but is not marked with the readonly trait

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