Malformed JSON requests currently reach generic crash handling and return 500 before any route handler runs. Classify the specific Express parser error as a 400 with a constant response, preserving unrelated server error reporting.
Carry forward the original three commits from #7410, preserve contributor credit, and add request privacy and negative regression checks. Document the API response. Fixes#7390.
Validation: 80 focused tests, direct server typecheck, and complete Linux CI passed. Greptile 5/5. Full local typecheck/build require missing Rust tooling; local test environment failures are documented in the PR.
Co-Authored-By: developers-universe-1 <madelynreyes2026@gmail.com>
Co-Authored-By: Paperclip <noreply@paperclip.ing>
