fix(api): return prompt error response instead of hanging when a SAS session fails

The SAS-runtime poll loop in processProgram only checked for
SessionState.completed, so a session that failed (e.g. via %abort;)
left the loop spinning on delay(50) forever - the request never
resolved, even though the SAS log had already been written. Split a single completed-on-either-outcome
flag into separate completed/failed states without updating this loop.

- processProgram now throws when session.state becomes 'failed'
- Execution.ts catches that, reads the complete log (guaranteed
  complete since the session's process has already exited by then),
  and throws a SessionExecutionError carrying it
- stp.ts/code.ts surface { ..., log } in the HTTP error response
- add unit tests for the poll loop and the log-enrichment logic, plus
  a mock SAS executable + end-to-end test exercising the real
  session/process pipeline without needing a SAS install
- add docs/diagrams covering the session lifecycle and the SAS
  execution handshake mechanism, for future context

api/src/controllers/code.ts

@@ -114,7 +114,8 @@ const executeCode = async (
       code: 400,
       status: 'failure',
       message: 'Job execution failed.',
-      error: typeof err === 'object' ? err.toString() : err
+      error: typeof err === 'object' ? err.toString() : err,
+      log: err?.log
     }
   }
 }