refactor(tests): extract shared buffer-cleanup and fake-vterm helpers

Eight test files across the ai-vterm and vterm-toggle suites each shipped a small variant of the same cleanup loop: walk `buffer-list`, kill any buffer whose name starts with a given prefix. Each file also re-implemented the `(string-prefix-p ...)` check inline. One file additionally had its own fake-vterm-mode-buffer constructor for tests that needed `cj/--vterm-toggle-buffer-p` to fire.

I pulled the shared logic into `tests/testutil-vterm-buffers.el`:

- `cj/test--kill-buffers-matching-prefix` is the primitive.
- `cj/test--kill-claude-buffers` and `cj/test--kill-test-vterm-buffers` are thin wrappers for the two prefixes that actually appear.
- `cj/test--make-fake-vterm-buffer` constructs a buffer with `major-mode` set to `vterm-mode` without launching a real vterm process.

Each affected test file now `(require 'testutil-vterm-buffers)` and calls the shared helpers directly. `test-vterm-toggle--buffer-filter.el` keeps a 3-line wrapper that calls both kill helpers in sequence (the only place that needs both prefixes). Net diff: -116 / +72 across 8 test files, plus ~30 lines in the new testutil. Roughly -45 lines after the abstraction is paid for.

No behavior change. 80 ai-vterm tests, 15 vterm-toggle tests, 15 cj-window-geometry tests all pass. Full make test green.

1 files changed, 38 insertions, 0 deletions

diff --git a/tests/testutil-vterm-buffers.el b/tests/testutil-vterm-buffers.el
new file mode 100644
index 00000000..864dd8f3
--- /dev/null
+++ b/tests/testutil-vterm-buffers.el
@@ -0,0 +1,38 @@
+;;; testutil-vterm-buffers.el --- Shared helpers for vterm/claude buffer tests -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Cleanup helpers and a fake-vterm constructor used across the
+;; ai-vterm and vterm-toggle test files.  Before this module, each
+;; test file re-implemented the same `(dolist (b (buffer-list))
+;; (when (string-prefix-p ...) (kill-buffer b)))' loop with a
+;; different prefix.
+
+;;; Code:
+
+(defun cj/test--kill-buffers-matching-prefix (prefix)
+  "Kill all live buffers whose name starts with PREFIX."
+  (dolist (b (buffer-list))
+    (when (string-prefix-p prefix (buffer-name b))
+      (kill-buffer b))))
+
+(defun cj/test--kill-claude-buffers ()
+  "Kill all live buffers whose name matches the AI-vterm prefix \"claude [\"."
+  (cj/test--kill-buffers-matching-prefix "claude ["))
+
+(defun cj/test--kill-test-vterm-buffers ()
+  "Kill all live buffers whose name starts with \"*test-vterm\"."
+  (cj/test--kill-buffers-matching-prefix "*test-vterm"))
+
+(defun cj/test--make-fake-vterm-buffer (name)
+  "Return a buffer named NAME with `major-mode' set to `vterm-mode'.
+
+Avoids actually launching a vterm process by setting the mode
+buffer-locally.  Used by tests that need a buffer satisfying the
+vterm-mode predicate without the side-effects of `(vterm)'."
+  (let ((buf (get-buffer-create name)))
+    (with-current-buffer buf
+      (setq-local major-mode 'vterm-mode))
+    buf))
+
+(provide 'testutil-vterm-buffers)
+;;; testutil-vterm-buffers.el ends here