[mujoco] Add native MetaWorld environments

[Trinkle23897]

Description

This adds a native C++ EnvPool implementation for the Farama MetaWorld v3 Sawyer manipulation benchmark, pinned to the official Farama-Foundation/Metaworld v3.0.0 source archive. The new family registers every ALL_V3_ENVIRONMENTS task under the MetaWorld/ prefix, keeps Meta-World/ aliases, wires the MetaWorld MuJoCo XML assets through third_party/metaworld_assets, and documents the supported task IDs and public space contract.

Key review points:

• envpool/mujoco/metaworld/metaworld_env.h contains the native task/state/reward implementation and all per-task variants.
• envpool/mujoco/metaworld/tasks.h centralizes the generated task metadata for all 50 v3 environments.
• envpool/mujoco/metaworld/metaworld_align_test.py reset-syncs once against the official MetaWorld oracle, then compares step-level observations, rewards, terminal flags, truncation flags, and exposed info for every task.
• envpool/mujoco/metaworld/metaworld_render_test.py covers reset and multi-step native rendering across the registered task set.
• docs/env/metaworld.rst adds the new family documentation and render sample.

Validation run:

• make ruff py-format
• make mypy
• make cpplint
• make clang-format
• make buildifier
• git diff --check
• bazel test --config=debug --test_output=errors //envpool/mujoco:metaworld_test //envpool/mujoco:metaworld_align_test //envpool/mujoco:metaworld_render_test
• devbox: make docstyle
• devbox: make bazel-test BAZEL_TEST_TARGETS=//envpool:make_test

Motivation and Context

MetaWorld is a common MuJoCo manipulation benchmark, but EnvPool did not have a native runtime implementation for it. This ports the full v3 task family without a Python runtime bridge, so users can run the benchmark through EnvPool's standard C++ backend, vectorized API, registry, docs, and packaging flow.

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• New environment (non-breaking change which adds 3rd-party environment)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of example)

Implemented Tasks

• Add native C++ MetaWorld v3 task implementations and pybind registration.
• Add official-oracle alignment, determinism, registry coverage, and render tests.
• Add third-party MetaWorld asset packaging and documentation.

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code using make format (required)
• I have checked the code using make lint (required)
• I have ensured make bazel-test pass. (required)

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reject unknown MetaWorld task names

GetTaskDef silently falls back to kMetaWorldTasks[0] when a task name is not found, which runs assembly-v3 instead of failing fast. A typo in task_name (or a future registration mismatch) will therefore execute the wrong environment while still reporting the requested ID, producing incorrect benchmark results that are hard to detect. This should raise an error/assert on unknown names instead of defaulting.

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