Add named scenarios to Context for flexible test state setup

[Copilot]

The Context class had its initial sample data hardcoded inline, with no way to switch between different pre-defined states at runtime.

Changes

• Extracted hardcoded data into a startup scenario — initial pets/users/orders now live in a top-level SCENARIOS constant; constructor calls this.loadScenario("startup") instead of inlining data
• Added three additional scenarios:
  • empty — no data; clean slate for building state from scratch
  • sold-out — all pets status: "sold", all orders complete: true
  • busy — 8 pets with mixed statuses, 3 users, 4 orders
• loadScenario(name) — replaces all context data and resets ID counters; callable from the REPL:

  ⬣> context.loadScenario("sold-out")
  ⬣> context.loadScenario("empty")
  

• context.scenarios getter — returns available scenario names for discovery in the REPL
• Scenarios deep-copy data on load so mutations between loads don't bleed through