Add mjs_makeFlex to the MuJoCo C and Python APIs.

- Add mjs_makeFlex C API that wraps mjCFlexcomp::Make(), creating
  a flex with auto-generated bodies, joints, and equality constraints
  from parameters (type, dim, dof, count, cellcount, spacing, scale,
  radius, mass, inertiabox, equality, rigid, flatskin, elastic2d,
  pos, quat, origin, file, vfs).
- Add Python binding body.make_flex() in specs.cc with keyword args.
- Refactor flex_spec_provision.py to use make_flex() instead of XML
  string templating + separate spec + attach. Eliminates textwrap
  dependency.
- Merge user_flexcomp into user_model BUILD target to resolve
  circular dependency.

PiperOrigin-RevId: 922725304
Change-Id: Ice13787c45f3173407fec659f9590a6453bc65dd

wasm/codegen/generators/functions.py

@@ -143,7 +143,7 @@ def get_param_string(p: ast_nodes.FunctionParameterDecl) -> str:
     # Pointer to primitive value parameters or arrays
     if p.type.inner_type.name == "char":
       if p.nullable:
-        return f"const NullableString& {p.name}"
+        return f"const StringOrNull& {p.name}"
       else:
         return f"const String& {p.name}"
     elif (