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/internal/Execution.ts

@@ -15,6 +15,24 @@ export interface ExecutionVars {
   [key: string]: string | number | undefined
 }
 
+// Thrown when the session itself fails (e.g. SAS exits abnormally via
+// %abort;). Carries the complete log - by the time this is thrown, the
+// session's process has already exited, so the log file it wrote is final,
+// not a partial/truncated snapshot.
+export class SessionExecutionError extends Error {
+  constructor(
+    message: string,
+    public log?: string
+  ) {
+    super(message)
+
+    // required for `instanceof` to work when compiling to ES5, since the
+    // default __extends helper does not preserve the prototype chain for
+    // classes extending built-ins like Error
+    Object.setPrototypeOf(this, SessionExecutionError.prototype)
+  }
+}
+
 export interface ExecuteReturnRaw {
   httpHeaders: HTTPHeaders
   result: string | Buffer
@@ -88,18 +106,24 @@ export class ExecutionController {
       preProgramVariables?.httpHeaders.join('\n') ?? ''
     )
 
-    await processProgram(
-      program,
-      preProgramVariables,
-      vars,
-      session,
-      weboutPath,
-      headersPath,
-      tokenFile,
-      runTime,
-      logPath,
-      otherArgs
-    )
+    try {
+      await processProgram(
+        program,
+        preProgramVariables,
+        vars,
+        session,
+        weboutPath,
+        headersPath,
+        tokenFile,
+        runTime,
+        logPath,
+        otherArgs
+      )
+    } catch (err: any) {
+      const log = (await fileExists(logPath)) ? await readFile(logPath) : ''
+
+      throw new SessionExecutionError(err.message, log)
+    }
 
     const log = (await fileExists(logPath)) ? await readFile(logPath) : ''
     const headersContent = (await fileExists(headersPath))