feat(stellar): implement circuit breaker for all Stellar RPC calls

[Obiajulu-gif]

Problem

This pr closes #611

All Stellar RPC calls (contract reads/writes, Horizon queries) were made without any circuit breaker. If the Soroban RPC endpoint degraded or became unresponsive, the server would continue making slow, blocking calls indefinitely — tying up resources and propagating failures to all dependents.

---

Solution

A custom CircuitBreaker class implementing the standard three-state pattern:

State	Behaviour
CLOSED	All calls pass through normally
OPEN	Calls are rejected immediately with CircuitOpenError
HALF_OPEN	One probe call is allowed to test recovery

State transitions are logged on every change:

[circuit:stellar-rpc] State transition: CLOSED → OPEN (will probe after 30000ms)
[circuit:stellar-rpc] State transition: OPEN → HALF_OPEN
[circuit:stellar-rpc] State transition: HALF_OPEN → CLOSED

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Changes

server/src/services/stellar-contract.service.ts

• Added CircuitState type, CircuitBreaker class, CircuitOpenError class
• Created singleton stellarRpcCircuitBreaker instance (configurable via env vars)
• Wrapped all 12 RPC-calling functions with stellarRpcCircuitBreaker.call()

server/src/routes/health.routes.ts

• Imported stellarRpcCircuitBreaker
• Added stellarRpc field to GET /api/health response:

{
  "stellarRpc": {
    "state": "CLOSED",
    "consecutiveFailures": 0,
    "openedAt": null
  }
}

---

Configuration

Env Var	Default	Description
STELLAR_CB_FAILURE_THRESHOLD	5	Consecutive failures before opening
STELLAR_CB_RESET_TIMEOUT_MS	30000	ms before probing recovery

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[Anuoluwapo25]

kindly fix conflicts.....And i noticed you don't give people issues on your repo...nice