400 errors on JSON uploads need clearer error messages

When uploading JSON for agent configs or test definitions, validation failures return a generic 400 with little context. Figuring out what actually broke turns into a binary search through the payload — slow and painful, especially with large configs.

What would help:

  1. Field-level error messages — name the offending key, the value, and the expected type/format (e.g. "tools[2].parameters.required must be an array of strings, received string")

  2. Multiple errors per response when there are several issues, so we fix everything at once instead of one round-trip per error

  3. Schema reference link in the error response pointing to the relevant docs section

This is standard practice for any API that accepts structured input — the current behavior forces us to guess and re-upload