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

---------

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

pyiceberg/table/update/snapshot.py

diff --git a/pyiceberg/table/update/snapshot.py b/pyiceberg/table/update/snapshot.py
index 37d1209..1df4e64 100644
--- a/pyiceberg/table/update/snapshot.py
+++ b/pyiceberg/table/update/snapshot.py
@@ -652,7 +652,7 @@
                 return [
                     ManifestEntry.from_args(
                         status=ManifestEntryStatus.DELETED,
-                        snapshot_id=entry.snapshot_id,
+                        snapshot_id=self._snapshot_id,
                         sequence_number=entry.sequence_number,
                         file_sequence_number=entry.file_sequence_number,
                         data_file=entry.data_file,

tests/integration/test_deletes.py

diff --git a/tests/integration/test_deletes.py b/tests/integration/test_deletes.py
index e3b487e..c49689b 100644
--- a/tests/integration/test_deletes.py
+++ b/tests/integration/test_deletes.py
@@ -975,3 +975,52 @@
     assert after_delete_snapshot is not None
 
     assert_manifest_entry(ManifestEntryStatus.DELETED, after_delete_snapshot.snapshot_id)
+
+
+@pytest.mark.integration
+def test_manifest_entry_snapshot_id_after_partial_deletes(session_catalog: RestCatalog) -> None:
+    identifier = "default.test_manifest_entry_snapshot_id_after_partial_deletes"
+    try:
+        session_catalog.drop_table(identifier)
+    except NoSuchTableError:
+        pass
+
+    schema = pa.schema(
+        [
+            ("id", pa.int32()),
+            ("name", pa.string()),
+        ]
+    )
+
+    table = session_catalog.create_table(identifier, schema)
+    data = pa.Table.from_pylist(
+        [
+            {"id": 1, "name": "keep"},
+            {"id": 2, "name": "keep"},
+            {"id": 3, "name": "delete_me"},
+            {"id": 4, "name": "delete_me"},
+        ],
+        schema=schema,
+    )
+    table.append(data)
+
+    # Partial delete: only some rows match, triggering CoW overwrite via _OverwriteFiles
+    table.delete(EqualTo("name", "delete_me"))
+    after_delete_snapshot = table.refresh().current_snapshot()
+    assert after_delete_snapshot is not None
+
+    manifests = after_delete_snapshot.manifests(table.io)
+    deleted_entries = [
+        entry
+        for manifest in manifests
+        for entry in manifest.fetch_manifest_entry(table.io, discard_deleted=False)
+        if entry.status == ManifestEntryStatus.DELETED
+    ]
+
+    assert len(deleted_entries) > 0, "Expected at least one DELETED manifest entry from the CoW overwrite"
+
+    for entry in deleted_entries:
+        assert entry.snapshot_id == after_delete_snapshot.snapshot_id, (
+            f"DELETED entry snapshot_id should be {after_delete_snapshot.snapshot_id} "
+            f"(the deleting snapshot), but was {entry.snapshot_id}"
+        )