fix: Information exposure through a stack trace (#10737)

Fix for
[https://github.com/Unleash/unleash/security/code-scanning/81](https://github.com/Unleash/unleash/security/code-scanning/81)

To prevent information exposure through stack traces, ensure that the
HTTP response sent to clients contains only sanitized, generic error
information, such as a status code and a simple message. Internal
details (including stack traces, error types, or internal error codes)
should not be sent to the client. These can be safely logged on the
server for debugging.

**The fix:**  
- Do not return the entire `finalError` object as JSON to the client, as
it may include fields like `stack` or `internalMessage`.
- Instead, return only a subset of fields that are safe to expose to the
user, in this case just `message` .
- Log the full error and any debugging details using the server-side
logger **as currently done**.


---
_Suggested fixes powered by Copilot Autofix. Review carefully before
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---------

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

src/lib/routes/util.ts

@@ -61,5 +61,23 @@ export const handleErrors: (
         );
     }
 
-    return res.status(finalError.statusCode).json(finalError).end();
+    // details property behaves weirdly. Trying to access it as finalError.details[0],
+    // hangs the execution of this method. Returning it as finalError.details doesn't
+    // work returning undefined. Printing out the finalError object using JSON.stringify
+    // shows that the details property is there and is an array.
+    // Running JSON.stringify(finalError.details) also hangs.
+    // As a workaround, we do a roundabout way of getting to the details property
+    // by doing JSON.parse(JSON.stringify(finalError))['details']
+    const details =
+        // @ts-expect-error - details might not be present on all UnleashErrors
+        // biome-ignore lint/complexity/useLiteralKeys: see above
+        finalError.details ?? JSON.parse(JSON.stringify(finalError))['details'];
+    return res
+        .status(finalError.statusCode)
+        .json({
+            name: finalError.name,
+            message: finalError.message,
+            details,
+        })
+        .end();
 };