fixed issue 618 642

[devmasalati]

closes #618
closes #642

Summary:

I have implemented a centralized timeout and retry policy framework for all external dependencies in the backend. This ensures that transient failures in services like Gmail, Outlook, Stellar, and various payment gateways do not cause cascading failures or hang the application.

Key Changes

Centralized Service Policies

Created external-services.ts which defines specific timeoutMs and retryPolicy (max attempts, backoff, jitter) for each provider.
Providers covered: gmail, outlook, stellar_rpc, stripe, paystack, exchange_rates, llm, slack, telegram, and outbound_webhooks.
Unified External Service Client

Implemented external-service-client.ts as the standard way to make external requests.
Features built-in AbortController for timeouts and exponential backoff via a centralized retry utility.
Automatically tracks in-memory metrics (total requests, successes, failures, timeouts, retries).
Service Migrations

Migrated core services to use the new client:
Email/OAuth: gmail-token-service.ts, gmail-service.ts, outlook-service.ts.
Payments: paystack.ts.
Rates: frankfurter-provider.ts, fiat-provider.ts, crypto-provider.ts.
Notifications: slack-service.ts, telegram-bot-service.ts.
Others: llm-parser.ts, webhook-service.ts.
Updated blockchain-service.ts and email-service.ts to align with the new policy constants.
Metrics & Monitoring

Updated monitoring-service.ts to aggregate external service metrics.
Exposed a new admin endpoint in index.ts: GET /api/v1/admin/metrics/external-services.
Documentation & Testing

Added EXTERNAL_SERVICE_POLICIES.md detailing the policy for each service.
Added unit tests in external-service-client.test.ts to verify timeout enforcement and retry behavior.
All automated tests passed, confirming that the new client correctly handles service timeouts and retries according to the defined policies.

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