PR 4721: Schema-based client uses modeled `Accept` header after switching protocols

[drganjoo]

Description

A client generated with RpcV2Cbor as its modeled protocol can select RestJson1 at runtime through the config bag. The resulting RestJson1 request incorrectly uses the modeled RpcV2Cbor Accept header:

accept: application/cbor

The multi-protocol Pokemon server rejects the request with HTTP 405.

Reproduction

The complete reproducer is on the upstream branch
fahadzub/4721-accept-error.

Fetch the branch and generate the Pokemon client and server:

git fetch origin fahadzub/4721-accept-error
git switch --detach origin/fahadzub/4721-accept-error
make -C examples

Start the server:

cd examples
cargo run --bin pokemon-service

In another terminal, run the client reproducer:

cd examples
cargo run --quiet \
    -p pokemon-service-client-usage \
    --example accept-header-repro

The example selects RestJson1 at runtime:

let config = Config::builder()
    .endpoint_url(POKEMON_SERVICE_URL)
    .protocol(
        aws_smithy_json::protocol::aws_rest_json_1::AwsRestJsonProtocol::new()
            .with_default_namespace("com.aws.example"),
    )
    .build();

Actual behavior

expected Accept="application/json"
actual Accept=Some("application/cbor")
result=Err(ServiceError(... StatusCode(405) ...))

Expected behavior

The runtime-selected RestJson1 protocol should send:

accept: application/json

The request should complete successfully.