chore(docs): add comprehensive fixture testing guidance to instructions

[devin-ai-integration]

Description

Devin Session: https://app.devin.ai/sessions/d14dcd6ae18740ceaa6ee41d3279a9e5
Requested by: Catherine Deskur (catherine@buildwithfern.com)

This PR adds comprehensive documentation about fixture testing requirements to help contributors understand when and how to include test fixtures with their code changes. The guidance emphasizes that fixture testing is a critical requirement for all code changes in the repository.

Changes Made

• Added new "Testing Changes with Fixtures" section to CLAUDE.md with detailed guidance on:
  • When to add/update fixtures (new features, bug fixes, behavior changes, IR changes, CLI features)
  • Step-by-step workflow for testing with fixtures
  • Best practices for fixture testing
  • Common testing patterns with examples
• Updated CONTRIBUTING.md with a concise "Important: Always Include Fixtures to Test Changes" section
• Added cross-references between the two documentation files
• Updated "Development Notes" section to reference the new fixture testing guidance

Testing

• Verified lint checks pass (pnpm run check)
• Reviewed documentation for accuracy against existing codebase structure
• Ensured commands and file paths match repository conventions
• Confirmed consistency with existing documentation style

Review Checklist

Please verify:

• Commands and file paths are accurate (e.g., /test-definitions/fern/apis/, seed/<generator>/seed.yml)
• Workflow steps match actual development practices in the repository
• Examples are realistic and helpful for contributors
• Guidance doesn't conflict with other documentation sections
• "When to add fixtures" scenarios cover the main use cases developers encounter

[devin-ai-integration]

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