Momo: Add mock endpoints supporting Orange Money callbacks

[sublime247]

Difficulty: Easy
Skills: TypeScript, Express, Mock servers
Files: src/mocks/providerMockServer.ts

Description:
Expand the local provider mock server to simulate Orange Money webhook callbacks for local testing.

Acceptance Criteria:

• Add /mock/orange/callback endpoint.
• Trigger mock successes/failures based on request headers.
• Unit tests cover the mocked responses.

Questions? Comment below!