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

[YuryShkoda]

Issue

Closes #388.

Stored Programs / code execution requests never respond when the underlying
SAS session fails (e.g. %abort;). The log file gets written up to the
point of failure, but the HTTP request just hangs forever — no crash, no
timeout, no response.

Intent

Make execution requests always resolve, whether the submitted code
completes normally or the session fails abnormally — surfacing a prompt
error response (with the complete SAS log attached) instead of hanging.

Implementation

Root cause: the SAS-runtime poll loop in processProgram.ts only
checked for SessionState.completed:

while (session.state !== SessionState.completed) {
  await delay(50)
}

It never checked for SessionState.failed, so once a session failed the
loop spun on delay(50) forever and the request never resolved.

Fix:

• processProgram.ts — throws when session.state becomes failed.
• Execution.ts — catches that around the processProgram call, reads the
  log (guaranteed complete at that point, since the session's process has
  already exited by the time failed is observable), and throws a new
  SessionExecutionError { message, log }.
• stp.ts / code.ts — both existing error-response catch blocks now
  include log in the HTTP error body alongside the pre-existing
  { code: 400, status: 'failure', message, error } shape.

Tests (no SAS runtime available, so built around a mock in place of one):

• processProgram.spec.ts — unit test on the poll loop directly. Genuinely
  reproduced the bug pre-fix: the "abort" case hit Jest's per-test timeout,
  and because the (buggy) loop keeps scheduling fresh timers, the whole
  Jest worker never exited on its own.
• Execution.spec.ts — unit test for the log-enrichment/error-wrapping
  logic (mocks processProgram, matches this repo's existing mocking
  convention from stp.spec.ts).
• routes/api/spec/files/mockSas.js — a minimal fake "SAS executable"
  (plain Node script) that fulfills just the SYSIN/AUTOEXEC filesystem
  handshake Session.ts relies on, without interpreting real SAS syntax.
  Validated directly via execFile before use.
• code.spec.ts — end-to-end test against POST /SASjsApi/code/execute
  (the endpoint Studio's "run code" hits) using the real, unmocked
  SASSessionController + processProgram pipeline wired to the mock
  executable. Covers both the happy path (200) and the regression case
  (%abort; → prompt 400 with the complete log, not a hang).

Full suite: 14/14 test suites, 242/242 tests passing. tsc --noEmit clean.

Docs: added api/docs/diagrams/ — Mermaid diagrams of the session
lifecycle, the SAS SYSIN/AUTOEXEC execution handshake, and the end-to-end
request flow, with embedded file:line references. Written for fast
context-loading (by humans or AI agents) on this otherwise non-obvious
subsystem; not specific to this bug.

Checks

• Code is formatted correctly (npm run lint:fix).
• Any new functionality has been unit tested.
• All unit tests are passing (npm test).
• All CI checks are green.
• Reviewer is assigned.

[github-actions]

🎉 This PR is included in version 0.39.5 🎉

The release is available on:

• GitHub release
• v0.39.5

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