Handle malformed JSON request bodies

[phucnguyen1707]

Fixes #6.

Summary

• Return HTTP 400 for malformed JSON request bodies instead of falling through to the 500 handler
• Keep the generic 500 handler for unexpected server errors
• Add a CommandBoard API contract regression test for malformed JSON submitted to /api/tasks

Tests

• npm --workspace @logicsrc/commandboard-api run test:contract
• npm run build