feat: add Fastify global setErrorHandler

[2witstudios]

Summary

• Adds a Fastify plugin (src/server/error-handler.ts) that registers a global setErrorHandler as a safety net for unhandled route errors
• PpgError instances map to their code and message with appropriate HTTP status
• Validation errors return { error: { code: "VALIDATION_ERROR", message } }
• Unexpected errors return { error: { code: "INTERNAL_ERROR", message } } — stack traces and internal details are suppressed in production (NODE_ENV=production)
• Uses fastify-plugin (via fp()) so the handler is registered at the root scope, not encapsulated

Test plan

• 9 tests covering PpgError, validation errors, unhandled errors (prod + dev), status code preservation, and JSON content-type
• npm test passes
• Verify integration with other Fastify server scaffold PRs once merged

Closes #87

Summary by CodeRabbit

• New Features

  • Implemented a new error handling system with structured error responses, HTTP status code mapping for custom error codes, and validation error handling with environment-specific message obfuscation.

• Tests

  • Added comprehensive test suite covering error handling across multiple scenarios, including status code mapping, validation errors, and environment-specific behavior.

• Chores

  • Added Fastify framework and plugin dependencies.

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📥 Commits

Reviewing files that changed from the base of the PR and between f250caa and dae3278.

📒 Files selected for processing (1)

• src/commands/spawn.test.ts

📝 Walkthrough

Walkthrough

This PR introduces a centralized error handling module for Fastify that registers a global error handler plugin. It implements HTTP status mapping for custom error codes, distinguishes between PpgError, validation errors, and unhandled errors, and provides environment-aware error responses. Two new Fastify dependencies have been added to support this functionality.

Changes

Cohort / File(s)	Summary
Dependencies package.json	Added Fastify 5.7.4 and fastify-plugin 5.1.0 dependencies.
Error Handler Implementation src/server/error-handler.ts	Introduced ErrorResponseBody interface, httpStatusForPpgError function for mapping custom error codes to HTTP statuses, errorHandler function handling PpgError/validation/unhandled errors, and a Fastify plugin export for global error registration.
Error Handler Tests src/server/error-handler.test.ts	Comprehensive test suite validating HTTP status mappings for specific error codes, validation error handling (400 responses), environment-specific unhandled error behavior (obfuscated in production), statusCode preservation, and content-type headers.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related issues

• Issue #87: The implementation directly fulfills the acceptance criteria for adding a Fastify global setErrorHandler, including structured JSON error responses, error logging, production-safe message handling, and comprehensive test coverage.

Poem

🐰 With whiskers twitched, the errors fly,
No more shall bad requests pass by,
A handler catches them with care,
Formats responses, clean and fair,
In JSON's nest, safe they rest! 🏠

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The PR title 'feat: add Fastify global setErrorHandler' clearly and specifically describes the main change: implementing a global error handler for Fastify.
Description check	✅ Passed	The description covers all required sections: What (adds error-handler plugin), Why (safety net), How (implementation details), and Checklist items addressing testing and code quality.
Linked Issues check	✅ Passed	The PR fulfills all acceptance criteria from #87: registers setErrorHandler, logs errors, returns structured JSON responses, suppresses production details, includes comprehensive tests, and handles PpgError/validation errors.
Out of Scope Changes check	✅ Passed	All changes are directly aligned with issue #87: new error-handler plugin with tests and package.json dependencies; no unrelated modifications present.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch ppg/issue-87-global-error

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