fix: hide Test ErrorBoundary button in production and improve claude CLI error handling

[roomote]

Summary

This PR fixes two issues reported in #6218:

1. Test ErrorBoundary button visible in production: The "Test ErrorBoundary" button in the About component was showing in production builds when it should only be visible during development.

2. Claude CLI command not found error: When using the claude-code provider without having the Claude CLI installed, users were getting an unhelpful ENOENT error.

Changes

1. Hide Test ErrorBoundary button in production

• Added a check for process.env.NODE_ENV !== "production" to conditionally render the Test ErrorBoundary button
• The button is now only visible during development

2. Improve Claude CLI error handling

• Added try-catch block around the execa call in runClaudeCode
• When an ENOENT error occurs (command not found), we now show a helpful error message with:
  • Clear indication that the Claude CLI is not found
  • Installation instructions
  • Link to the Claude CLI repository
  • Information about specifying a custom path in settings

3. Added tests

• Added comprehensive tests for the new ENOENT error handling
• Tests cover both default and custom path scenarios
• Tests ensure non-ENOENT errors are rethrown unchanged

Testing

• All existing tests pass ✅
• New tests added for error handling ✅
• Manually tested that the Test ErrorBoundary button is hidden in production builds
• Type checking passes ✅
• Linting passes ✅

Fixes #6218

---

Important

Hide "Test ErrorBoundary" button in production and improve runClaudeCode error handling for missing Claude CLI.

• Behavior:
  • Hide "Test ErrorBoundary" button in About.tsx during production by checking process.env.NODE_ENV.
  • Improve error handling in run.ts for runClaudeCode by catching ENOENT errors and providing a helpful message.
• Testing:
  • Add tests in run.spec.ts for ENOENT error handling, covering default and custom path scenarios.
  • Ensure non-ENOENT errors are rethrown unchanged.

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[daniel-lxs]

Duplicate #6215