Data storage `content`: Base64 validity is not expressed by the schema (error 202 trigger)

[lorandmolnar]

Context

POST /masters/{masterNumber}/ports/{portNumber}/datastorage declares
error 202 (JSON data value invalid), and a genuine trigger exists: a
content string of valid length that is not decodable Base64 cannot
encode any DataStorageObject. The schema currently expresses only

type: string
minLength: 16
maxLength: 2748

so the Base64 requirement lives in prose only, and the invalid-value
case cannot be derived from the schema.

The JSON Test Specification already generates the corresponding
conformance test case
(TCJ_ERRS_CONTENT_POST_PORT_DS_202_NOT_BASE64, defined via a
supplement trigger constraint) on the assumption that the expected
answer is 400 with error code 202. Tracking: register entry ISS-25
(doc/open_issues.md).

Request

1. Confirm that a non-Base64 content value shall be rejected with
   error 202.
2. Add the Base64 constraint to the schema so the case becomes
   schema-derivable, e.g.

pattern: '^[A-Za-z0-9+/]*={0,2}$'

(or format: byte).