fix(api): return 200 with embedded log on SAS session failure instead of 400

A failed session (e.g. SAS %abort;, or a non-zero JS/PY/R exit) is a
normal outcome of running arbitrary user code, not a request-shape or
server problem. The #388 fix stopped processProgram() from hanging
forever on a failed SAS session, but did so by throwing a
SessionExecutionError, which surfaced as an HTTP 400 with a bespoke
JSON shape - breaking Studio's log tab, which only renders on 2xx.

Align the SAS branch with the pre-existing JS/PY/R behaviour: resolve
instead of throwing, and let Execution.ts fold session.failureReason
into the log the same way it already does for a debug-mode run. This
removes the now-dead SessionExecutionError class and its try/catch
wrapper entirely.

Update the diagrams in api/docs/diagrams/ to match.

api/src/controllers/code.ts

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