Fix #3104: include file-referenced assets in MjSpec.to_zip

[Ashutosh0x]

Problem

MjSpec.to_zip only serializes assets present in the Python-side spec.assets dict. Assets referenced via XML file attributes — e.g. <mesh file="cube.obj"/>, <texture file="tex.png"/> — are loaded by the C layer from disk but are never added to spec.assets, so they are silently omitted from the zip. The resulting zip cannot be reloaded.

Fixes #3104.

Changes

python/mujoco/__init__.py

• After copying spec.assets, walk all spec elements that can carry file references: meshes, hfields, skins, and textures (including cubefiles).
• For each referenced file not already present in the dict, resolve the path against spec.modelfiledir and read it from disk. Files not found on disk are silently skipped (in-memory assets already in spec.assets take precedence).
• Two secondary bugs also fixed:
  • spec.assets was mutated in-place (files_to_zip = spec.assets then items were added to it); now a copy is made first so the caller's dict is untouched.
  • os.makedirs(directory, ...) was called even when directory is '' (e.g. saving to current directory), which raises FileNotFoundError; now guarded with if directory.

python/mujoco/specs_test.py

• test_to_zip_includes_file_assets: loads testdata/msh.xml (which references abdomen_1_body.msh by file path), calls to_zip, and asserts the mesh file is present in the zip. Also verifies the zip round-trips to a compilable spec.
• test_to_zip_does_not_mutate_assets: asserts spec.assets content is unchanged after to_zip.

Reproducer

import io
import mujoco

spec = mujoco.MjSpec.from_file("model_with_mesh.xml")
buf = io.BytesIO()
mujoco.to_zip(spec, buf)
buf.seek(0)
spec2 = mujoco.from_zip(buf)
model = spec2.compile()  # previously raised -- now works