Implement Soroban Provider SLA Compliance Reports

# 🧠 Concept
Generate SLA compliance reports for providers.

## ⚠️ Problem
Provider reliability commitments are difficult to evaluate.

## 📁 Implementation Scope
- `src/reporting/sla/providers/stellar/`

## ✅ Requirements
- Measure uptime
- Measure response times
- Generate compliance reports

## 🎯 Acceptance Criteria
- [ ] SLA reporting implemented
- [ ] Compliance reports available