Refactor: Group autogenerated structs source in bindings.cc

This change modifies the code generation for structs in `bindings.cc`. Instead of placing each struct's constructor and member functions under individual markers, all autogenerated struct source code is now grouped together under a single `// {{ AUTOGENNED_STRUCTS_SOURCE }}` marker. This improves the organization of the generated file and simplifies the template.

Future work would move the rest of the structs into this pattern

PiperOrigin-RevId: 828390272
Change-Id: Iee9d0e8766db86b2c1d4296a37ac7a8e6e05428b

wasm/codegen/helpers/structs.py

@@ -592,6 +592,16 @@ def build_struct_source(
     wrapped_fields: List[WrappedFieldData],
 ):
   """Builds the C++ .cc file code for a struct."""
+  # These structs require specific function calls for creation and/or deletion
+  # which, for now, are hardcoded in the template file.
+  if struct_name in [
+      "mjData",
+      "mjModel",
+      "mjvScene",
+      "mjSpec",
+  ]:
+    return ""
+
   s = struct_name
   w = common.uppercase_first_letter(s)
   is_mjs = "Mjs" in w