Consider deprecating old validation API

**Problem**
When working with middleware, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

No files, tests, or entry points are named. Start by clarifying which validation API is considered old, where middleware validation and error handling are implemented, and what compatibility policy is expected. Done should include an agreed deprecation scope, more descriptive errors, and tests covering the resulting behavior.

Written by the indexing model from the issue text.