fix(problem)!: adding a problem code no longer changes an API's OpenAPI types

[giraffesyo]

Summary

humaproblem.Document listed every code as an enum on Problem.code and FieldError.code. That made each new code a change to the OpenAPI document and to SDKs generated from it: strictly typed clients see a new enum value, and every branch that adds a code edits the same lines of a checked-in spec.

RFC 9457 treats problem types as an open set, and clients here already fall back to the status for a code they don't know. So code is now an open string, documented as such, and the catalog at /problems/ remains the list of codes.

Breaking: humaproblem.Document is removed. Delete the call; nothing replaces it.

Testing

• make check
• TestSchemaLeavesCodesOpen checks neither schema has an enum and the default response is still problem+json.

Checklist

• make check passes
• A new or changed shared message is in every language in packages/problem/src/messages/ (none changed)
• No code, rule or param was renamed (they are API contract)