Document why restart-preserves-default-directory test skips in batch

The `agent-shell-restart-preserves-default-directory' test calls
`make-frame' on a hidden frame to drive an interactive restart flow,
which fails with "Unknown terminal type" under emacs --batch.
Upstream guards the body with `(skip-unless (not noninteractive))';
add a comment above it explaining the constraint so a future reader
doesn't mistake the skip for an unfinished test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timvisher-dd

tests/agent-shell-tests.el

@@ -2601,6 +2601,8 @@ Based on ACP traffic from https://github.com/xenodium/agent-shell/issues/415."
 After `kill-buffer' happens during restart, Emacs falls back to another
 buffer.  Without the fix, `default-directory' would be inherited from
 that fallback buffer, potentially starting the new shell in the wrong project."
+  ;; `make-frame' below requires a real terminal, so this test cannot
+  ;; run in batch mode where Emacs has no controlling terminal.
   (skip-unless (not noninteractive))
   (let ((shell-buffer nil)
         (other-buffer nil)