Add skill: mapbox-testing-patterns

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## Summary

Testing Mapbox applications is a gap with no existing skill. AI assistants don't know how to mock the map, assert layer state, or set up CI/CD for style validation. This leads to untested map code or brittle tests that depend on live network calls.

## Topics to cover

- **Unit testing** - mocking `mapboxgl.Map` in Jest/Vitest, testing layer/source logic without rendering
- **Integration testing** - headless map rendering in CI (Playwright, Puppeteer), asserting visual state
- **iOS testing** - XCTest patterns for map views, mocking map delegates
- **Android testing** - Espresso/Compose UI test patterns for map views
- **Style CI/CD** - running `validate_style_tool` and `check_color_contrast_tool` in CI pipelines
- **Snapshot testing** - capturing and diffing map renders across style changes

## Key patterns a skill should teach

- How to mock `mapboxgl.Map` without breaking the module graph
- What to assert vs. what to trust the SDK to handle
- Setting up a headless Chromium environment for map screenshot tests
- When to use the MCP DevKit validation tools in CI vs. manual review

## Why this matters

Map code often goes untested because developers don't know how to approach it. A skill here would meaningfully improve code quality across the ecosystem.

Start by defining the scope of the mapbox-testing-patterns skill around the listed unit, integration, iOS, Android, CI/CD, and snapshot topics. Research mocking mapboxgl.Map, headless Playwright or Puppeteer rendering, XCTest and Espresso patterns, and the named validation tools. Done means the skill covers each requested testing pattern with actionable guidance and CI considerations.

Written by the indexing model from the issue text.