Add kitless core environment test config factories

[nvsekkin]

Description

Split the fixture foundation out of #6526 so later core-test changes can reuse empty environment configurations without importing task or integration-scene dependencies.

• Add config-only factories for empty manager-based, manager-based RL, and direct MARL environments.
• Add a fresh-process CPU canary that constructs and validates every factory without loading task packages, assets, Kit/runtime roots, SciPy, or integration-scene-only modules.

Type of change

• Test infrastructure (non-breaking)

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[greptile-apps]

Greptile Summary

This PR introduces a minimal, kitless foundation of shared environment config factories (ManagerBasedEnvCfg, ManagerBasedRLEnvCfg, DirectMARLEnvCfg) and a fresh-process canary test that verifies none of the forbidden runtime or integration-scene modules are pulled in during import or construction.

• source/isaaclab/isaaclab/test/env_cfgs.py adds three factory functions and two supporting @configclass helpers (EmptyManagerCfg, EmptySceneCfg) that rely only on core isaaclab.envs, isaaclab.sim, isaaclab.scene, and isaaclab.utils.configclass.
• source/isaaclab/test/envs/test_env_cfgs.py adds a single pytest test that spawns a fresh subprocess, constructs all three configs with device=\"cpu\", calls validate(), asserts basic field values, then checks sys.modules for a set of forbidden roots and prefixes.
• A .skip changelog marker correctly labels this as test-only infrastructure with no user-facing API changes.

Confidence Score: 4/5

The change is self-contained test infrastructure with no effect on production code paths; the main concern is that the canary subprocess has no timeout and may stall CI if something goes wrong during module initialization.

Both changed source files are test-only. The factory module imports core env configs but deliberately avoids integration-scene and runtime dependencies — the approach is sound and the module-boundary canary is a good pattern. The two findings are both non-blocking: a missing subprocess timeout that could cause CI to hang indefinitely rather than fail quickly, and literal values in the MARL factory that silently diverge from the shared constants used by the other two factories.

The MARL factory in source/isaaclab/isaaclab/test/env_cfgs.py and the subprocess.run call in source/isaaclab/test/envs/test_env_cfgs.py both deserve a second look.

Important Files Changed

Filename	Overview
source/isaaclab/changelog.d/core-test-config-fixtures.skip	Changelog skip marker for a test-only change; content is correct and no issues found.
source/isaaclab/isaaclab/test/env_cfgs.py	Adds three kitless environment config factories and two supporting configclasses; the MARL factory uses hardcoded decimation/episode_length values inconsistently with the module-level constants used by the other two factories.
source/isaaclab/test/envs/test_env_cfgs.py	Fresh-process canary test verifying kitless imports and forbidden-module boundaries; subprocess.run lacks a timeout so the test can hang indefinitely if the spawned process deadlocks.

_{Reviews (1): Last reviewed commit: "Add kitless core environment fixtures" | Re-trigger Greptile}