Support cross-entity checkpoint forks

[poofeth]

Fixes #649.

Summary

• Add an optional from_entity parameter to ServerlessBackend._experimental_fork_checkpoint.
• Use the explicit source entity when building the W&B artifact collection path, falling back to the destination model entity and then W&B default entity.
• Add unit coverage for explicit source entity, fallback behavior, and missing-entity validation.

Validation

• uv run pytest tests/unit/test_serverless_fork_checkpoint.py -q
• uv run ruff check src/art/serverless/backend.py tests/unit/test_serverless_fork_checkpoint.py
• uv run ruff format --check src/art/serverless/backend.py tests/unit/test_serverless_fork_checkpoint.py
• git diff --check

Note: this is a regular issue fix, not an Algora bounty claim.

[poofeth]

Added a direct fake-W&B regression test in 6f9280c2 to prove from_entity is threaded into the artifact lookup call.

Additional validation after the update:

uv run pytest tests/unit/test_serverless_fork_checkpoint.py -q
uv run ruff check src/art/serverless/backend.py tests/unit/test_serverless_fork_checkpoint.py
uv run ruff format --check src/art/serverless/backend.py tests/unit/test_serverless_fork_checkpoint.py
git diff --check

Result: 5 tests passed; lint, format-check, and diff-check passed.

[poofeth]

Closing this to keep my open review queue focused on active bounty work. This is a regular issue fix rather than an Algora/bounty claim, and I do not want it to add noise while prioritizing payout-backed PRs. The branch and tests remain available if maintainers want to reuse the patch later.