[Fix] Publish and fetch the checkpoints a task's components declare

[hujc7]

Summary

play --checkpoint pretrained crashed on the Shadow Hand camera tasks: the vision CNN the policy
needs was never published beside it, and nothing in the tooling knew the file existed. This PR adds
the smallest mechanism that fixes it — a component declares what it writes, and the tooling
publishes and fetches it with the policy.

The refactor that this mechanism invites is deliberately not here; it is stacked in
#7509 — Replace the pretrained checkpoint functions with CheckpointBundle.

Description

1. Declaring a checkpoint

A component declares what it writes on its own config:

@configclass
class FeatureExtractorCfg:
    checkpoint: Checkpoint = Checkpoint(name="feature_extractor", run_glob="cnn_*.pth")
    """The trained CNN, published beside the policy checkpoint."""

get_declared_checkpoints walks the resolved environment config to find every declaration, so a
task declares nothing and the component that writes the file owns its name. Each file is published
beside the policy as <policy stem>_<name><extension> and fetched with it. Checkpoint.resolve
hands the component its local file, so no component needs to know the naming convention.

Checkpoint also covers weights that already exist (url=): a frozen encoder is fetched but never
published by the checkpoint tooling.

2. Published checkpoints

Both companion pairs are trained and published, so the fix has something to fetch:

Task	Backends
Isaac-Reorient-Cube-Shadow-Camera	newtonmjwarp+newton, physx+rtx
Isaac-Reorient-Cube-Shadow-Camera-Direct	newtonmjwarp+newton, physx+rtx

Each is a policy .pt plus its _feature_extractor.pth.

3. Verification

• 82 tests pass across test_pretrained_checkpoint.py, test_checkpoints.py,
  test_train_and_publish_checkpoints.py, test_shadow_hand_camera_presets.py and
  test_pretrained_checkpoint_lookup.py. isaaclab -f clean.
• The declaration tests pin discovery through nested component configs, exclusion of url weights,
  and that a published file keeps the extension the component declared.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Release backport

• Backport this pull request to the active release branch after it merges into develop

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 added a changelog fragment under source/<pkg>/changelog.d/ for every touched package
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[hujc7]

run-ci

[hujc7]

run-ci

[hujc7]

run-ci

[hujc7]

run-ci

[hujc7]

run-ci

[hujc7]

run-ci

[hujc7]

run-ci