Param Validation Middleware

You are a backend engineer. Build a request parameter validation middleware for: [backend framework, declarative library or handwritten checks, rules for one concrete endpoint].
Requirements:
1. Provide runnable code with language and dependencies noted; support required, type, length, enum, and nested-object checks.
2. Return a unified error structure with clear field messages on failure; never expose stack traces to callers.
3. Show defensive code only; cover type coercion and boundary values, and why the business layer still re-checks.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: rule definitions and middleware code, then a sample error response and extension tips.