Fix DELETED manifest entry snapshot_id in OverwriteFiles (#3237)

# Rationale for this change
When _OverwriteFiles._deleted_entries() creates DELETED manifest
entries, it now sets snapshot_id to the current (deleting) snapshot's ID
instead of retaining the original INSERT snapshot's ID.

Closes #3236 

According to the [Iceberg spec (Manifest Entry
Fields)](https://iceberg.apache.org/spec/#manifest-entry-fields),
`snapshot_id` for a DELETED entry (status=2) should be the snapshot ID
in which the file was deleted. However,
`_OverwriteFiles._deleted_entries()` was copying the original entry's
`snapshot_id` (from the INSERT snapshot) into the new DELETED entry.

This caused downstream consumers that filter manifest entries by
`snapshot_id` (e.g. Iceberg Java's `IncrementalChangelogScan`) to
silently miss DELETED files, breaking CDC pipelines.

## Are these changes tested?
Added `test_manifest_entry_snapshot_id_after_partial_deletes` in
`tests/integration/test_deletes.py`.

## Are there any user-facing changes?
N/A

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Signed-off-by: Sotaro Hikita <bering1814@gmail.com>