Add from_entity parameter to _experimental_fork_checkpoint

Problem

_experimental_fork_checkpoint in ServerlessBackend constructs the source artifact path using the destination model's entity (line ~89):

from_entity = model.entity or api.default_entity
collection_path = f"{from_entity}/{from_project}/{from_model}"

This means you can't fork from a checkpoint in a different W&B entity. For example, forking from willow-voice/willow_normal/kl-000-1 into wb-training/willow_normal/my-new-run fails because it looks for the artifact under wb-training.

Proposed Fix

Add an optional from_entity parameter to _experimental_fork_checkpoint:

async def _experimental_fork_checkpoint(
    self,
    model: Model,
    from_model: str,
    from_project: str | None = None,
    from_entity: str | None = None,  # NEW
    from_s3_bucket: str | None = None,
    not_after_step: int | None = None,
    verbose: bool = False,
    prefix: str | None = None,
) -> None:

And use it when constructing the artifact path:

from_entity = from_entity or model.entity or api.default_entity

Current Workaround

Download the artifact from the source entity and re-upload it to the destination entity before calling fork. This works but doubles the artifact storage and adds an unnecessary copy step.

Start at ServerlessBackend._experimental_fork_checkpoint and inspect the artifact path construction around line 89. Add the optional source-entity input described in the issue, and verify that a checkpoint can be forked from an entity different from the destination model's entity without the download-and-reupload workaround.