Consider ErrorHandlingMiddleware turning arbitrary errors into JSON error bodies

### Motivation

See the discussion on this thread, we left it out of the implementation of SOAR-0011: https://github.com/apple/swift-openapi-runtime/pull/126#discussion_r1836133413

### Proposed solution

We should consider it as an opt-in feature, similar behavior to Vapor (which enables it for debug only automatically).

### Alternatives considered

_No response_

### Additional information

_No response_

Start with the discussion in SOAR-0011 and the linked pull request, especially discussion_r1836133413, then review how Vapor handles arbitrary errors and debug-only behavior. The issue does not name files or tests; completion would require an agreed opt-in design for ErrorHandlingMiddleware to produce JSON error bodies.

Written by the indexing model from the issue text.