fix: remove corporate-specific URLs from test fixtures

[CodeWithJuber]

What & why

Replace employer-specific gateway URLs and scenario names in test/providers.test.js with generic example.com equivalents. Five occurrences of a corporate domain were present in test fixtures from the provider-detection work in PR #44 — this removes them for privacy.

• https://api-eu1.aigateway..group → https://api.aigateway.example.com
• "scenario" / "env" → "corporate gateway scenario" / "corporate gateway env"

Checklist

• npm test passes (Node 18/20/22)
• npm run check passes (Biome lint + format)
• New public functions have a test
• Conventional commit message (feat:/fix:/docs: …)
• CHANGELOG.md updated under ## [Unreleased]
• No new runtime dependency (dev deps ok)
• Substrate/docs updated if this changes forge substrate, forge impact, router/gate, or MCP substrate tools

Risk & rollback

• Risk level: low
• Rollback plan: revert the single commit — test URLs are cosmetic, no behavior change

Extra checks (tick if applicable)

• npm run typecheck passes
• Logs contain no secrets/PII
• If AI-assisted: I understand it, verified the package APIs, and it has tests

---

Generated by Claude Code