Feat/oracle health check

[Luchistack]

#closes #908
Title: feat: add oracle health-check endpoint and documentation

Description:
This PR introduces a /health endpoint to the oracle service, enabling real-time monitoring of connectivity to the Stellar network and external dependencies. This ensures higher availability and better observability for the platform's infrastructure.

Key Changes:

Implementation: Added a new /health GET route in oracle-service/src/main.rs that returns the service status, network, contract address, and a timestamp.

Documentation: Updated docs/oracle.md to include endpoint documentation, request/response examples, and status code information.

Testing:

[x] Verified response structure and JSON formatting.

[x] Ensured standard HTTP 200 response for service availability.

#closes

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