test: add a regression test for lean_apply_m over-application

Applies 20 arguments in one call to a chain of arity-1 closures, which segfaults without the `lean_apply_m` fix.

Author: Kha

tests/compile/apply_m_overapp.lean

@@ -0,0 +1,22 @@
+/-!
+This is a regression test reproducer for #14969.
+
+Applying more than `closureMaxArgs` arguments at once to a closure of smaller arity used the
+array calling convention, which is only correct above that arity, and crashed. `Chain` keeps
+every closure at arity 1, and `f` is opaque in `apply20` so that all 20 arguments are applied
+in a single `lean_apply_m` call.
+-/
+
+abbrev Chain : Nat → Type
+  | 0     => Nat
+  | n + 1 => Nat → Chain n
+
+def mk : (n : Nat) → Chain n
+  | 0     => 42
+  | n + 1 => fun _ => mk n
+
+@[noinline] def apply20 (f : Chain 20) : Nat :=
+  f 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
+
+def main : IO Unit :=
+  IO.println (apply20 (mk 20))

tests/compile/apply_m_overapp.lean.out.expected

@@ -0,0 +1 @@
+42