[next.js] Malformed percent-encoding in an App Router dynamic segment returns 500 instead of 400

## Observed symptom Malformed percent-encoding in an App Router dynamic segment returns 500 instead of 400 Link to the code that reproduces this issue https://github.com/y-pakorn/next-malformed-param-500 ## Literal error text ``` returns 404, so only dynamic segments are affected. ### Provide environment information ``` ## Environment GitHub vercel/next.js #99347 ## Trigger / context Public GitHub report in vercel/next.js (FRESH_7D). ## Source evidence - https://github.com/vercel/next.js/issues/99347 (2026-09-28T07:42:41Z) — evidence_class: PRIMARY_ISSUE_REPORT — #99347 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:50:53.329738Z - scout_id: C0e3cb8cda1 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.