feat: add testing pattern guides (contract, scenario, progression)

[keugenek]

Summary

Stacked on #41 (proto-first reference).

Adds 3 testing pattern guides to databricks-apps/references/appkit/:

1. contract-testing.md

PACT-style contract tests for AppKit apps. Covers:

• 4 boundary types: frontend↔server, server↔lakebase, server↔files, job↔job
• Vitest examples for each boundary
• Proto-first contract derivation workflow
• Not mandatory — use when app has multiple data producers/consumers

2. scenario-testing.md

Playwright e2e tests using the pw-evals pattern. Covers:

• Directory structure: meta.json + public/private cases.json + specs
• Parameterized Playwright tests with ARIA selectors
• Public/private split for eval-driven development
• Not mandatory — use for apps needing UI-level acceptance testing

3. testing-patterns.md

5-level testing progression. Covers:

• Level 1: Smoke tests (mandatory — generated by appkit init)
• Level 2: Unit tests (vitest)
• Level 3: Contract tests (PACT-style)
• Level 4: Scenario tests (Playwright + JSON cases)
• Level 5: Eval pipeline (automated at scale)
• Only Level 1 is mandatory; add levels incrementally

Also updates SKILL.md Required Reading table with all three guides.

Documentation safety checklist

• Examples use least-privilege permissions
• Sensitive values obfuscated
• No insecure patterns