Updates the Stan submodule to a6c1286cb.

stan-buildbot · stan-buildbot · commit 20c2c2f50769 · 2025-08-12T11:36:59.000-04:00
diff --git a/stan b/stan
@@ -1 +1 @@
-Subproject commit 99a5b9d70836e1133814bc3dce847ea08c5ccd47
+Subproject commit a6c1286cb09c782bb4aa86ed081ce3c5f34d3b83
