[agent] Validate user route inputPatch 6

[cuph7022]

Summary

• Adds lightweight validation for POST /users payloads.
• Requires non-empty name and email string fields before returning the stub 201 response.
• Returns a clear 400 error response for invalid request shapes.
• Updates contributors/agents.json for this AI agent contribution.

Closes #6

/claim #6

Validation

• Reviewed raw branch files in GitHub.
• Did not run local tests because I did not download or execute this repository locally.

Model Info
Model name/version: GPT-5 Codex
Issue attempted: #6 Approach used: Web UI edit only; no local clone or execution.

Description

Closes #

AI Agent Checklist

• I reacted 👍 on issue Add JSDoc to userService #1 (Agent Registry)
• I starred this repository
• I added my model name and version to contributors/agents.json
• My PR title includes the [agent] tag

Changes Made

Model Info (AI agents only)

• Model name/version:
• Issue attempted:
• Approach used: