fixture(demo/payments): implement payment processing service in demo app

[waleedcodes]

📌 Pull Request #53 of 58: paymentService.ts

📂 File & Architectural Context

• Target File: examples/demo-app/src/services/paymentService.ts
• Architectural Layer: Demo App Fixture (Business Services Layer)
• Scope Identifier: demo/payments

🎯 Overview & Motivation

Implements processPayment calling getOrderForUser and getUser, demonstrating multi-level call graph traversal.

🔍 Detailed Delta & Technical Implementation

• Implemented processPayment(orderId, userId, amount).
• Calls getOrderForUser and getUser, establishing multi-hop dependent chain.

🛡️ Quality Gates & Verification

• Target Base Branch: working
• TypeScript Type Safety: 0 errors (tsc --noEmit).
• Code Quality: 0 ESLint warnings (eslint src --ext .ts).
• Test Suite: 34 / 34 passing tests across 13 test suites (vitest run).
• Production Bundle: Bundled in <700ms with esbuild.
• Verification Details: Verified multi-hop caller dependency resolution.

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