feat: expose per-endpoint circuit state for RPC calls

[ZacLou]

Summary

Addresses #630

Problem

Consumers had no way to check if an RPC endpoint was degraded without catching the thrown 'circuit open' error. There was no read-only state inspection.

Solution

src/rpc-circuit-state.ts (new)

A circuit breaker module with:

• getCircuitState(scope) — returns { state, failureCount, lastFailureAt, lastSuccessAt, cooldownUntil }. State is 'closed' (healthy), 'open' (degraded, exceeding threshold), or 'half-open' (probing after cooldown).
• recordSuccess(scope) — resets failure count, closes circuit
• recordFailure(scope, options?) — increments failure count, opens circuit at threshold (default 5)
• resetCircuit(scope) — clears all state
• getAllCircuitStates() — returns all known circuit states for dashboards

src/soroban.ts

The createRpcServer proxy now calls recordSuccess(rpcUrl) on successful calls and recordFailure(rpcUrl) when the retry logic exhausts or hits a non-retriable error.

src/index.ts

Exports getCircuitState, recordSuccess, recordFailure, resetCircuit, getAllCircuitStates, CircuitState, CircuitStatus.

Usage

import { getCircuitState } from '@conduit-protocol/sdk';

const state = getCircuitState(rpcUrl);
if (state.state === 'open') {
  showBanner('RPC endpoint is degraded. Retrying...');
}

Verification

• Typecheck passes
• All 989 tests pass (78 test files)