feat(bucket-object): add delete-marker flows [13/n]

[sadhansood]

Description

This adds delete-marker support to bucket_object.

The main changes are:

• ObjectVersion can now represent a blob-less delete marker via new_delete_marker
• BucketObject adds delete_object and delete_object_if_match
• finalize_pending_version_if_certified now promotes pending delete markers without requiring blob certification
• put_object_if_absent_and_register now treats a current delete marker as absent so keys can be recreated after deletion

This keeps the model consistent with the existing object-version flow: deletes are just another version transition, not a separate special-case object state.

Test plan

• rustup run 1.86 cargo fmt -- --config group_imports=StdExternalCrate,imports_granularity=Crate,imports_layout=HorizontalVertical
• cargo clippy --workspace --all-features --tests --no-deps -- -D warnings
• sui move test --path /private/tmp/walrus-bucket-object-13/contracts/bucket_object

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