Updates the Stan submodule to db9851c79.

stan-buildbot · stan-buildbot · commit 0e3dd7a97dd8 · 2026-01-29T05:42:55.000-05:00
diff --git a/StanHeaders/inst/include/upstream b/StanHeaders/inst/include/upstream
@@ -1 +1 @@
-Subproject commit d13c50c0fe7e4a4c387b727b81246cee2541a320
+Subproject commit db9851c7969e9d363100b20e826acbd4521329e8
