Return error object when headers already sent in response-validation handler

[Copilot]

Addresses inconsistency in response-validation.js where the handler returned undefined when headers were already sent, unlike other error handlers (unauthorized.js, request-validation.js, not-found.js) which always return the error response object.

Changes

• Removed early return when response.headersSent at function start
• Modified validation error handling to construct error object unconditionally
• Conditionally send response via response.status(502).json(errorResponse) only when headers not sent
• Return error object in all cases to allow downstream inspection of intended response

Before:

if (!response.headersSent) {
  return response.status(502).json({ errors, status: 502, ... })
}
return undefined  // Inconsistent with other handlers

After:

const errorResponse = { errors, status: 502, ... }
if (!response.headersSent) {
  return response.status(502).json(errorResponse)
}
return errorResponse  // Consistent with other handlers

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[Copilot]

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