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-commands.el b/tests/test-ai-config-commands.el
index 8da2e4b01..fed06d82b 100644
--- a/tests/test-ai-config-commands.el
+++ b/tests/test-ai-config-commands.el
@@ -86,7 +86,7 @@ globally and reports via `message'."
          added)
     (unwind-protect
         (cl-letf (((symbol-function 'featurep)
-                   (lambda (sym) (not (eq sym 'projectile))))
+                   (lambda (sym &rest _) (not (eq sym 'projectile))))
                   ((symbol-function 'read-file-name)
                    (lambda (&rest _) target))
                   ((symbol-function 'gptel-add-file)
@@ -133,7 +133,7 @@ globally and reports via `message'."
     (cl-letf (((symbol-function 'gptel-context-remove-all)
                (lambda () (setq called t)))
               ((symbol-function 'call-interactively)
-               (lambda (fn) (funcall fn)))
+               (lambda (fn &rest _) (funcall fn)))
               ((symbol-function 'message)
                (lambda (fmt &rest args) (setq msg (apply #'format fmt args)))))
       (cj/gptel-context-clear))