Add CCX creation event signal to edx-platform

Context

CCX creation is a significant lifecycle event that often requires integration with external systems or platform-specific workflows.

Today, edx-platform does not expose a clear, dedicated signal that indicates when a CCX has been created.

Problem statement

Currently, there is no first-class signal emitted specifically for CCX creation. As a result:

Integrations must rely on:

• Ad-hoc hooks

• Overloaded signals

• Or duplicating logic across multiple code paths

There is no consistent or reliable way to react to CCX creation across:

• Extensions

• Plugins

• External services

This makes CCX lifecycle integrations brittle and difficult to maintain.

Proposal

Introduce a dedicated signal that is emitted when a CCX is successfully created.

The signal should:

• Fire exactly once per CCX creation

Be emitted from a single, canonical location in the CCX creation flow

This would allow extensions and plugins to reliably subscribe to CCX creation events without duplicating logic.

Why this is viable

• Low risk: signals are passive and do not alter core behavior

• Backward compatible: no existing functionality is removed

• Extensible: enables integrations without hardcoding platform-specific logic

• Aligns with existing patterns: edx-platform already uses signals for other course lifecycle events

Additional context

A draft implementation has been proposed in:
https://github.com/openedx/edx-platform/pull/37897

Start by reviewing the draft implementation in PR 37897 and tracing the CCX creation flow it changes. Done means a dedicated signal is emitted exactly once from one canonical creation location, with coverage for that behavior.