fix: preserve provider pins in slot defaults

[PaulFidika]

What changed

• Preserve Civitai version in slots[].default_checkpoint.
• Preserve Hugging Face and ModelScope revision in slots[].default_checkpoint.
• Add an end-to-end discovery contract test covering the binding and slot-default representations.
• Bump gen-worker to 0.25.1.

Why

The binding serializer already emitted provider-specific immutable pins, but the slot-default serializer dropped them. Tensorhub therefore received a pinned function binding alongside an unpinned default checkpoint, which could mirror or resolve a different upstream model version.

Impact

First-publish slot seeding and runtime fallback now refer to the same immutable upstream artifact.

Validation

• uv lock --check
• mypy src/gen_worker — 114 files clean
• pytest tests/test_slot_discovery.py tests/test_drain_flush.py -q — 10 passed
• CPU-equivalent full suite — 1,057 passed, 26 skipped
• uv build — sdist and wheel built successfully