fix: expand cookbook error output tail from 12 to 50 lines (#1538)

* fix: expand cookbook error output tail from 12 to 50 lines

When a task reaches status 'error', the status endpoint was returning
only the last 12 lines of the subprocess log. The existing context-menu
'Copy last 50 lines' action was therefore copying the same 12 lines,
making it useless for diagnosing failures that produce long stack traces
or build output.

- Set _tail_lines = 50 when status == 'error', keep 12 for running tasks
- Initialise exit_code = None before the status-classification block so
  it is always defined in the result dict (was only set inside the
  is_alive branch, potential NameError in the dead-session path)
- Include exit_code in the task-status response dict
- JS poller captures exit_code from live data into local task state

The frontend output panel and 'Copy last 50 lines' now show the actual
error context without any UI changes.

* refactor: extract output-tail logic to testable helper + behavioral tests

Addresses review feedback on #1538: the previous tests were source-level
string guards. Extract the tail-slicing into a dependency-free helper
(routes/cookbook_output.error_aware_output_tail) and replace the guards
with behavioral tests that exercise the actual logic:

- error status with a 200-line snapshot -> exactly the last 50 lines
- running/ready/completed/stopped/unknown -> last 12 lines
- short snapshot -> all lines, no padding
- empty snapshot -> empty string
- error tail is a strict superset (suffix-compatible) of the non-error tail

The helper has no FastAPI/SQLAlchemy imports so it unit-tests without
standing up the app.

---------

Co-authored-by: Alexandre Teixeira <111787685+alteixeira20@users.noreply.github.com>

static/js/cookbookRunning.js

@@ -3547,6 +3547,7 @@ async function _pollBackgroundStatus() {
           updates.status = live.status === 'ready' ? 'ready' : 'running';
         }
         if (live.progress && live.progress !== task.progress) updates.progress = live.progress;
+        if (live.exit_code != null && live.exit_code !== task.exit_code) updates.exit_code = live.exit_code;
         if (live.output_tail) {
           const previous = String(task.output || '');
           const tail = String(live.output_tail || '');