Introduce mjPreContact, a minimal struct passed into the collision functions.

PiperOrigin-RevId: 918533795
Change-Id: I2b5af05c1479b25d5c2cfdc690321a26fce6ede6

wasm/codegen/generators/constants.py

@@ -292,6 +292,7 @@ STRUCTS_TO_BIND: list[str] = list(
 NO_DEFAULT_CONSTRUCTORS: tuple[str, ...] = (
     # go/keep-sorted start
     "mjContact",
+    "mjPreContact",
     "mjSolverStat",
     "mjStatistic",
     "mjTimerStat",

wasm/codegen/generators/structs.py

@@ -493,7 +493,7 @@ def build_struct_source(
 
   if not is_mjs:
     # default constructor
-    with builder.function(f"{w}::{w}() : ptr_(new {s}){fields_init}"):
+    with builder.function(f"{w}::{w}() : ptr_(new {s}()){fields_init}"):
       builder.line("owned_ = true;")
       default_func = _default_function_statement(s)
       if default_func: