feat: add phase 1 Datadog backend instrumentation for thread creation

[santhosh-apphelix-2u]

Summary

This PR adds the forum-side backend instrumentation needed for Phase 1 of the thread creation telemetry flow.

Phase 1 is intentionally focused on a single vertical slice so the end-to-end Datadog behavior can be validated in stage before broader rollout.

What Changed

• added forum-side backend context emission during backend selection
• emits:
  • forum.backend

Why

In the current LMS discussion flow:

• request-level telemetry belongs in edx-platform
• backend execution context belongs in forum

This PR covers the forum-owned part of that split by attaching backend selection context to the same trace.

Validation

This PR should be validated together with the paired edx-platform and edx-internal changes.

Expected Datadog outcome:

• thread creation trace includes forum.backend
• backend context appears on the same LMS trace as the request-level attributes

Related

• Ticket: COSMO2-853