[Feature] Add LBForaging environment wrapper

[Iliamsou]

Description

Adds a native TorchRL wrapper for the Level-Based Foraging (LBF)
multi-agent Gymnasium environment.

Files added/modified:

• torchrl/envs/libs/lbforaging.py (new): LBForagingWrapper + LBForagingEnv
• torchrl/envs/libs/__init__.py, torchrl/envs/__init__.py: export the new classes
• docs/source/reference/envs_libraries.rst: doc entry
• test/libs/test_multiagent.py: TestLBForaging (8 tests — env construction for multiple configs, wrapper construction from a pre-built env, per-agent reward/done shapes, truncation vs termination across an episode boundary, collector integration)

Motivation and Context

LBF is a widely used cooperative MARL benchmark for credit assignment;
TorchRL currently has no wrapper for it.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/rl/4301

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure

As of commit ace8c20 with merge base b0eab87 ():

NEW FAILURE - The following job has failed:

• Unit-tests on Linux / tests-cpu (3.13, bulk) / linux-job (gh)
  test/objectives/test_dreamer_v3.py::TestDreamerV3CompileStrategy::test_stepper_defaults_follow_the_device

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[vmoens]

LGTM Thanks!