Request validation against a JSON-Schema subset, plus a middleware that rejects malformed request bodies with 422.

validate/2 checks a decoded JSON term (maps with binary keys, lists, binaries, numbers, booleans, null) against a schema map.

Supported keywords:

• core: type (single or a list of types), enum, const
• numbers: minimum, maximum, exclusiveMinimum, exclusiveMaximum, multipleOf
• strings: minLength, maxLength, pattern
• objects: required, properties, additionalProperties (false or a schema), minProperties, maxProperties
• arrays: items, minItems, maxItems, uniqueItems
• combinators: allOf, anyOf, oneOf

Schema keys may be atoms or binaries; property names inside properties must be binaries (they are matched against decoded JSON keys). It is a pragmatic subset, not a complete JSON Schema implementation ($ref, if/then/else, patternProperties, and dependentSchemas are not supported).

The call/3 middleware reads #{body_schema => Schema} from its state, decodes the request's JSON body, validates it, and on failure short-circuits with a 422 whose body lists the errors. On success it stores the decoded body under meta(body, Decoded).