Omit `throws` on `json` computed var when possible

### Motivation

Currently, a computed var `json` is generated for every REST response body. Unfortunately, this var is marked as `throws` even if `application/json` is the only content type defined for that response body. This is the case for 100% of our REST API methods, and I imagine is the most common case across all users of this library.

```swift
/// The associated value of the enum case if `self` is `.json`.
///
/// - Throws: An error if `self` is not `.json`.
/// - SeeAlso: `.json`.
public var json: Operations.postLinkedAccounts.Output.Conflict.Body.jsonPayload {
get throws {
switch self {
case let .json(body):
return body
}
}
}
```

This means that at the call site, I have to use `try!` to do something that is in reality perfectly safe:

```swift
case .conflict(let error): throw .conflict(try! error.body.json)
```

### Proposed solution

The generator should only add the `throws` annotation to the computed var `json` if in fact there are multiple cases/content types for that response body.

### Alternatives considered

_No response_

### Additional information

_No response_

Search the generator for the computed `json` property and its `get throws` emission, then inspect tests covering response-body accessors. Reproduce the single-content-type case and compare it with a response having multiple content types; done means only the former omits `throws` and the existing generation tests pass.

Written by the indexing model from the issue text.