fix(examples): run HTTP demos against local demo server

[Kewe63]

Summary

Fixes #88

The HTTP examples currently rely on https://x402-demo.sapiom.ai, but that public demo host can return Cloudflare 403s and make the examples fail before users can verify the SDK integration.

This PR makes the axios, fetch, and node-http examples self-contained by running them against a local demo server by default.

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Changes

• Add examples/local-demo-server.mjs with the endpoint shapes used by the HTTP examples:
  • GET /api/public/time
  • GET /api/public/status
  • GET /api/crm/customers
  • POST /api/sms
  • POST /api/campaigns/analytics
• Add examples/run-local-demo.mjs to start the local server, set DUMMY_SERVER_URL=http://localhost:3101, run the selected example script, and shut the server down.
• Update axios/fetch/node-http npm start, npm run free, and npm run full scripts to use the local demo helper.
• Update .env.example and README docs so the broken public demo host is no longer the default.
• Add troubleshooting guidance for x402-demo.sapiom.ai 403s.

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How to Test

corepack pnpm examples:check:test
corepack pnpm examples:check
node --check examples/local-demo-server.mjs
node --check examples/run-local-demo.mjs
git diff --check

npm start     # examples/axios
npm start     # examples/fetch
npm start     # examples/node-http
npm run full  # examples/axios
npm run full  # examples/fetch
npm run full  # examples/node-http

✅ All passed locally.

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Compatibility

• No SDK API changes.
• No dependency changes.
• Example-only behavior change: HTTP examples now use a local demo server by default.
• Users can still override DUMMY_SERVER_URL to point at another compatible server.

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Checklist

• Tests pass — all example scripts and validation checks green
• Follows Conventional Commits
• Changes scoped to this fix only
• No SDK API or dependency changes

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Risk & Impact

Low. Example-only change — no SDK code, API surface, or dependencies are touched. Users who already override DUMMY_SERVER_URL are unaffected; the change only alters the default target for the HTTP examples.

Type: 🐛 Bug fix
Fixes: #88