Declare the error contract, streaming endpoints, and chat pagination

[ExtraToast]

What to build

agents-api's contract gains its missing error codes, its four undeclared real-time endpoints, and a pagination convention for its unbounded collections.

agents-api is the best of the three today — 8 error responses across 52 operations, and it already emits ProblemDetail/FieldError — but four endpoints that agents-ui depends on in production appear nowhere in the spec: two EventSource streams, one fetch-reader stream, and a WebSocket. Nothing gates changes to their payload shapes.

Acceptance criteria

• Universal error responses declared, plus the domain-specific codes this service actually returns
• The three HTTP streaming endpoints appear in the spec; the WebSocket is documented where OpenAPI cannot express it
• Chat sessions and messages paginate using the estate cursor convention
• oasdiff breaking changes are intentional and called out, not incidental

Blocked by

• Declare ProblemDetail responses on every operation (kotlin-spring-commons tracker)