test: make subr mocks variadic for native-comp, add arity meta-test

Re-enabling native-comp surfaced a suite-wide fragility. When a test redefines a C primitive (or a native-compiled function), native-comp routes native callers through a trampoline that calls the mock with the primitive's maximum arity. A fixed-arity mock narrower than the primitive then throws wrong-number-of-arguments, intermittently, as the eln-cache fills.

I swept every arity-narrow subr mock to append &rest _ (188 sites, preserving any named args the body uses), and added tests/test-meta-subr-mock-arity.el, which fails make test on any subr mock too narrow for the primitive's arity. The rule isn't "never mock a subr". The suite mocks message and completing-read freely. It's "a subr mock must accept the primitive's arity."

Background, the three failure modes, and the research are in docs/native-comp-subr-mocking.org.

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/test-ai-config-gptel-commands.el b/tests/test-ai-config-gptel-commands.el
index 371a75cc..cab23572 100644
--- a/tests/test-ai-config-gptel-commands.el
+++ b/tests/test-ai-config-gptel-commands.el
@@ -128,7 +128,7 @@
 (ert-deftest test-ai-config-add-this-buffer-calls-gptel-add-with-prefix ()
   "Normal: `cj/gptel-add-this-buffer' calls `gptel-add' with the (4) prefix arg."
   (let ((arg nil))
-    (cl-letf (((symbol-function 'featurep) (lambda (_) t))
+    (cl-letf (((symbol-function 'featurep) (lambda (_ &rest _) t))
               ((symbol-function 'gptel-add)
                (lambda (a) (setq arg a)))
               ((symbol-function 'message) #'ignore))
@@ -144,7 +144,7 @@
          (deleted nil))
     (unwind-protect
         (cl-letf (((symbol-function 'get-buffer-window)
-                   (lambda (_b) 'fake-window))
+                   (lambda (_b &rest _) 'fake-window))
                   ((symbol-function 'delete-window)
                    (lambda (w) (setq deleted w))))
           (cj/toggle-gptel))