fix(server): add safe correlationId fallback in global error handler

[Joyyyb]

Issue closed: safe correlation ID fallback in global error handler

Problem

The global error handler (app.use((err, req, res, next) => { ... })) in stellar-payment-platform/server.js logged and returned req.correlationId directly. If an error is thrown before the correlation-ID middleware has had a chance to attach that property to the request object, the value is undefined. This caused console.error to print '[Trace ID: undefined]' and the JSON response body to contain { trace_id: undefined }, which is stripped by JSON serialisation, leaving clients with no trace reference at all.

Root Cause

No defensive guard existed around req.correlationId before it was used. Express does not guarantee that every middleware has run by the time an error propagates to the error-handling layer, so req.correlationId can legitimately be absent.

Fix Applied (stellar-payment-platform/server.js, ~line 372) ------------------------------------------------------------ Added a single fallback assignment immediately before the value is used:

const traceId = req.correlationId || 'unknown';

Then updated both call sites that previously referenced req.correlationId:

1. console.error([Error ID: ${errorId}] [Trace ID: ${traceId}], err);
2. JSON response body now includes: trace_id: traceId

This means:

• When the correlation middleware has already run, the real request ID is preserved and surfaced as before.
• When the middleware has not yet run (or the property is falsy for any other reason), the string 'unknown' is used instead of undefined, keeping log lines and API responses consistent and parseable.

No other logic was changed. The existing reference_id (crypto.randomUUID) that uniquely identifies each error occurrence is retained alongside the new trace_id field.

Acceptance Criteria Met

• The global error handler safely defaults to 'unknown' when req.correlationId is falsy.
• console.error and the JSON response body both use the same traceId variable, ensuring they are always in sync.

Closes #237

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[Abdulazeem-code]

Fix backend build test error