Community Sample: NEX Agent Co. — first live A2A + x402 dual-protocol on Base

Hi A2A team — would like to add [NEX Agent Co.](https://nexaitechau.github.io/) as a community A2A sample that also implements the x402 extension end-to-end.

**What NEX ships:**
- A2A JSON-RPC 2.0 endpoint at /a2a/tasks/send (1000 free calls/IP/day for crawlers, then x402 USDC at $0.001-0.01)
- 11 paid x402 USDC endpoints (chat, code, code-pro, vision, embed, crypto-price, weather, geo, fx, whois)
- 10 free mirrors at /v1/free/* (30/min/IP)
- Apache-2.0 reference server: github.com/NEXAITECHAU/nex-x402-server (FastAPI + Ollama + x402 v2)
- 23 on-chain NFTs (Identity, Pass, Attestation, Receipt) on Base for ERC-8004-style reputation
- 17 settle_ok events via PayAI facilitator

**Why NEX is a fit for this samples repo:**
- NEX is the first live A2A + x402 dual-protocol agent in production
- Implements payment-required → payment-submitted → payment-completed flow
- Uses a2a-samples style of /a2a/tasks/send JSON-RPC 2.0
- Open-source Apache-2.0 reference for anyone to fork
- Real on-chain settlements on Base

**Discovery:**
- A2A card: https://nexaitechau.github.io/agent-card-a2a.json
- x402 manifest: https://charm-preparing-avon-ips.trycloudflare.com/.well-known/x402.json
- Server card: https://charm-preparing-avon-ips.trycloudflare.com/.well-known/agent.json
- Live: https://nexaitechau.github.io/live.html

**Open question:**
Should I (a) add a samples/python/nex/ subdirectory following the existing pattern, (b) add a samples/community/ section in README.md, or (c) file a sample in a different language? Happy to ship a clean implementation that uses the official a2a-python SDK + x402-a2a extension.

Reference: already indexed by google-agentic-commerce/a2a-x402#171 (issue), xpaysh/awesome-x402, Merit-Systems/awesome-agentic-commerce, bitrefill/awesome-agentic-payments.

Look at the existing samples/python/ directory structure to understand the pattern. The task is to add a new sample subdirectory for NEX Agent Co., likely under samples/python/nex/. Review the provided GitHub repo (github.com/NEXAITECHAU/nex-x402-server) to see the FastAPI + Ollama + x402 v2 implementation. Determine if a README update or a new sample file is needed, and ensure it follows the a2a-samples style for JSON-RPC 2.0 endpoints.

由索引模型根據 Issue 內容生成。