[openapi to tsp] Fix `accept` header logic

Given the following swagger definition:
```
"produces": [
"application/vnd.microsoft.appconfig.kv+json",
"application/problem+json"
],
```

The converter outputs the following typespec code where all of the supported content types are bundled into 1 string:
```
accept: "application/vnd.microsoft.appconfig.kv+json, application/problem+json";
```

The correct code should represent each content type individually:
```
accept: "application/vnd.microsoft.appconfig.kv+json" | "application/problem+json";
```

cc @joheredi

Start with the OpenAPI-to-TypeSpec converter logic that handles the Swagger `produces` field and maps it to the `accept` header. Verify the generated TypeSpec for multiple content types, and consider the work complete when each supported type is emitted as a separate union member rather than one comma-separated string.

Written by the indexing model from the issue text.