fix(core): report an unknown events() port as ImposterNotFound

[EtaCassiopeia]

Fixes #254

Problem

RemoteTransport.events(options) treated every 404 from GET /events as "this engine has no admin event stream" and threw UnsupportedOperationException. The engine also answers 404 when options.port() names no imposter, so a wrong port was reported as an engine that is too old.

Change

• On a 404 from /events with port(...) set, the SDK now calls GET /imposters/{port}, the same lookup every other per-port call makes, and maps the result the same way:
  • The imposter is missing: ImposterNotFound(port, "Imposter not found on port N").
  • The imposter exists: the 404 really means there is no stream, so UnsupportedOperationException as before.
  • Without a port, behaviour is unchanged.
• The body isn't used to tell these apart. As the issue notes, an engine without the route and an engine after rift#1226 both answer with the no such resource envelope.
• extractErrorMessage now also reads the bare {"error":"..."} body that engines up to 0.18.1 use for /events refusals. A bad types filter now reports its message instead of the raw JSON.
• The Rift.events javadoc has a new @throws ImposterNotFound; docs/events.md and docs/design/sdk-api.md are updated to match.

Validation

• Engine check against rift 0.18.1:
  • /events?port=<unknown> → 404 {"error":"no imposter on port N"}
  • /imposters/<unknown> → 404 envelope Imposter not found on port N
  • an unknown route → 404 envelope Not Found
  • bad types → 400 {"error":"unknown types value ..."}
• Unit tests (EventStreamTest), faked in both body shapes:
  • unknown port with the 0.18.1 body → ImposterNotFound
  • unknown port with the new envelope → ImposterNotFound
  • a 404 for an existing port → still unsupported
  • bare {"error"} 400 → InvalidDefinition carrying the message
• Live IT (EventStreamIT): events(port(1)) → ImposterNotFound. It passed on SPAWN (rift 0.18.1 binary) and EMBEDDED (JDK 25).
• ./mvnw -B verify is green on JDK 21, and javadoc:javadoc is clean.

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