fix(api-v3): log the traceback on the editor state-write failure

The 848 fix answers 500 when the session state cannot be written, and logged
that at error level -- but without exc_info, so the traceback never reached the
log. test_web_error_detail.py guards exactly this: a handler returning 5xx must
write an error-level record *with* the traceback and return the sanitized
detail, because checking that merely something was logged is too weak.

Caught by Core unit tests on the previous commit, not locally: the guard parses
every module under web_interface/blueprints/api_v3 as one source, so it only
fires once the whole package is read together.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/starlark.py

@@ -872,7 +872,11 @@ def start_pixlet_editor():
                 # The process is up but nothing records its PID: status and stop
                 # would both report no session while the display stays down until
                 # the timeout expires. Take the editor with us instead.
-                logger.error('Started an editor session but could not record it: %s', err)
+                # exc_info: this path answers 500, and the guard in
+                # test_web_error_detail.py requires the traceback to reach the log
+                # as well as the sanitized detail reaching the caller.
+                logger.error('Started an editor session but could not record it: %s',
+                             err, exc_info=True)
                 _terminate_editor_process(process.pid)
                 return jsonify({'status': 'error',
                                 'message': 'Could not record the editor session; '