fix(handlers): return computed error status/message instead of generic 500

[ankit25bcs10610]

What

Four handlers — completionsHandler, embeddingsHandler, imageGenerationsHandler, imageEditsHandler — compute the correct error status and message in their catch block, then throw it away:

let statusCode = 500;
let errorMessage = 'Something went wrong';

if (err instanceof RouterError) {
  statusCode = 400;
  errorMessage = err.message;
}

return new Response(
  JSON.stringify({
    status: 'failure',
    message: 'Something went wrong', // ⬅️ hardcoded, ignores errorMessage
  }),
  {
    status: 500,                     // ⬅️ hardcoded, ignores statusCode
    ...
  }
);

So a conditional-routing failure (RouterError) that should return 400 + the real reason instead returns an opaque 500 "Something went wrong", making these failures hard to diagnose for callers.

Fix

Use the already-computed errorMessage and statusCode in the response. This matches the sibling handlers that already do the right thing: chatCompletionsHandler, messagesHandler, messagesCountTokensHandler, proxyHandler.

8-line change across the four files; no behavior change for non-RouterError errors (still 500 + "Something went wrong"). npm run format:check passes.