[CT-27] Write integration tests for GET /health

[mftee]

Problem

The /health endpoint checks Stellar and Redis connectivity but has no automated test coverage. A broken health endpoint would silently report incorrect status to load balancers and monitoring systems.

Proposed Solution

Write integration tests for the health check endpoint inside contract/module/tests/health_tests.rs.

Acceptance Criteria

• Test: when both StellarClient.check_connection() and CacheBackend.check_connection() return true the endpoint returns 200 with status: healthy
• Test: when StellarClient.check_connection() returns false the endpoint returns 200 with status: degraded and stellar_connected: false
• Test: when CacheBackend.check_connection() returns false the endpoint returns 200 with status: degraded and redis_connected: false
• Test: when both checks fail the endpoint returns 200 with status: degraded and both connected fields as false
• Tests use mock implementations of StellarClient and CacheBackend to control check_connection return values
• All test files live inside contract/module/tests/