[envpool] auto-register minigrid environments

[Trinkle23897]

Summary

• Problem: the top-level envpool import path never loaded envpool.minigrid.registration, so MiniGrid envs were documented and individually registered but absent from envpool.list_all_envs() and failed in envpool.make(...).
• Scope: wire MiniGrid into the top-level registration and build targets, and add a focused regression test for the normal make path.
• Outcome: MiniGrid envs are available from a standard import envpool again, and make_gymnasium("MiniGrid-Empty-5x5-v0") succeeds.

This fixes #311.

Technical Details

• Approach: import envpool.minigrid.registration from envpool.entry, then mirror that dependency in Bazel so the top-level envpool target keeps the registration module and extension available.
• Code pointers:
  • envpool/entry.py: adds MiniGrid to automatic registration on import envpool
  • envpool/BUILD: adds MiniGrid registration/runtime deps to the top-level target
  • envpool/make_test.py: adds targeted regression coverage for MiniGrid creation
• Notes: keeps the existing generic test_list_all_envs unchanged and puts the MiniGrid assertion in its own test.

Test Plan

Automated

• brix ssh dev-0 -C -- 'chmod +x /tmp/envpool_issue311_fix_verify.sh && bash /tmp/envpool_issue311_fix_verify.sh /tmp/envpool_issue311_fix_repo.tar.gz': passed; //envpool:make_test ran _MakeTest.test_list_all_envs and _MakeTest.test_make_minigrid successfully on Linux/Python 3.12

Suggested Manual

• python -c 'import envpool; print("MiniGrid-Empty-5x5-v0" in envpool.list_all_envs())': verify the task is registered from the top-level import path
• python -c 'import envpool; envpool.make("MiniGrid-Empty-5x5-v0", env_type="gymnasium", num_envs=2)': verify the original issue reproducer now succeeds