feat: add configurable webhook latency simulation to mock provider server

[MJ-RWA]

Changes Made

Added src/config/mockServer.ts with webhookLatencyMs (default 3000ms) and webhookLatencyEnabled (default true) config values driven by environment variables
Added a delay helper in scripts/provider-mock-server.ts that returns a Promise via setTimeout
Wrapped all webhook callback dispatch points with the configurable delay before firing
Added MOCK_WEBHOOK_LATENCY_MS and MOCK_WEBHOOK_LATENCY_ENABLED to .env.example with explanatory comments
Updated integration tests in tests/integration/ to accommodate delayed callbacks — increased jest timeouts to 15000ms and added a waitForWebhook polling helper where needed

Testing

Ran integration tests with MOCK_WEBHOOK_LATENCY_ENABLED=true and MOCK_WEBHOOK_LATENCY_MS=3000 — callbacks arrived after ~3 seconds and tests passed
Set MOCK_WEBHOOK_LATENCY_ENABLED=false — callbacks fired instantly, confirming the flag works correctly
Set MOCK_WEBHOOK_LATENCY_MS=5000 — confirmed delay is respected and configurable without code changes
Ran full test suite — no regressions

Closes: #860

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