fix(python): preserve mixed blob values before Arrow conversion (#4304)

## Cause
Python list and pandas rows were converted to Arrow before the target
blob v2 schema was applied. In a mixed bytes and URI batch, Arrow
inferred a binary column and encoded the URI as inline bytes; dict and
`lance.Blob` values could fail conversion.

## Fix
Build blob v2 columns from the original Python values using the target
field's storage type before converting the rest of each row. This
preserves inline bytes, external URIs, descriptor dictionaries, blob
objects, and nulls for `add`, `merge_insert`, and `create_table`.

## Validation
- `python/tests/test_blob.py`: 76 passed, 1 skipped.
- Python project Ruff check passed.
- Repository-wide Ruff check still reports 29 existing findings in `ci/`
files outside this change.

Fixes #4271

<!-- lance-gatekeeper-fix:v1 agent=54c95f06f11c88187b07a430aac6be2f
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/tests/test_blob.py

@@ -7,10 +7,11 @@ import sys
 import textwrap
 
 import lance
+import pandas as pd
 import pyarrow as pa
 import pyarrow.compute as pc
 import pytest
-from lance.blob import BlobType as LanceBlobType
+from lance.blob import Blob, BlobType as LanceBlobType
 
 import lancedb
 from lancedb._blob import (
@@ -1432,3 +1433,129 @@ def test_add_external_uri_string_round_trips_with_flag(tmp_path):
     hits = table.search().to_arrow()
     blobs = table.fetch_blobs("image", hits)
     assert blobs[0].as_py() == payload
+
+
+@pytest.mark.parametrize("as_pandas", [False, True])
+def test_add_bytes_and_uri_in_one_batch_preserves_external_uri(tmp_path, as_pandas):
+    payload = b"external-payload"
+    blob_path = tmp_path / "payload.bin"
+    blob_path.write_bytes(payload)
+    rows = [
+        {"id": 1, "image": b"inline"},
+        {"id": 2, "image": blob_path.as_uri()},
+    ]
+    db = lancedb.connect(tmp_path / "db")
+    schema = pa.schema([pa.field("id", pa.int64()), lancedb.blob("image")])
+    table = db.create_table("mixed", schema=schema)
+
+    table.add(
+        pd.DataFrame(rows) if as_pandas else rows,
+        allow_external_blob_outside_bases=True,
+    )
+
+    by_id = _row_ids_by_id(table)
+    assert table.fetch_blobs("image", [by_id[1], by_id[2]]).to_pylist() == [
+        b"inline",
+        payload,
+    ]
+
+
+@pytest.mark.parametrize("as_pandas", [False, True])
+def test_add_mixed_python_blob_values_preserves_external_uris(tmp_path, as_pandas):
+    payload = b"external-payload"
+    blob_path = tmp_path / "payload.bin"
+    blob_path.write_bytes(payload)
+    uri = blob_path.as_uri()
+    rows = [
+        {"id": 1, "image": b"inline"},
+        {"id": 2, "image": uri},
+        {"id": 3, "image": {"data": b"from-dict"}},
+        {"id": 4, "image": Blob.from_bytes(b"from-blob")},
+        {"id": 5, "image": Blob.from_uri(uri)},
+        {"id": 6, "image": {"uri": uri}},
+        {"id": 7, "image": None},
+    ]
+    db = lancedb.connect(tmp_path / "db")
+    schema = pa.schema([pa.field("id", pa.int64()), lancedb.blob("image")])
+    table = db.create_table("mixed", schema=schema)
+
+    table.add(
+        pd.DataFrame(rows) if as_pandas else rows,
+        allow_external_blob_outside_bases=True,
+    )
+
+    by_id = _row_ids_by_id(table)
+    assert table.fetch_blobs("image", [by_id[i] for i in range(1, 8)]).to_pylist() == [
+        b"inline",
+        payload,
+        b"from-dict",
+        b"from-blob",
+        payload,
+        payload,
+        None,
+    ]
+
+
+@pytest.mark.parametrize("as_pandas", [False, True])
+def test_merge_insert_mixed_python_blobs_validates_external_uri(tmp_path, as_pandas):
+    blob_path = tmp_path / "payload.bin"
+    blob_path.write_bytes(b"external-payload")
+    table = _blob_table("merge_mixed_uri", [{"id": 1, "image": b"before"}])
+    rows = [
+        {"id": 1, "image": b"updated"},
+        {"id": 2, "image": blob_path.as_uri()},
+    ]
+
+    with pytest.raises(ValueError, match="allow_external_blob_outside_bases"):
+        (
+            table.merge_insert("id")
+            .when_matched_update_all()
+            .when_not_matched_insert_all()
+            .execute(pd.DataFrame(rows) if as_pandas else rows)
+        )
+    assert table.count_rows() == 1
+
+
+@pytest.mark.parametrize("as_pandas", [False, True])
+def test_merge_insert_python_blob_dicts_and_objects(as_pandas):
+    table = _blob_table("merge_python_blobs", [{"id": 1, "image": b"before"}])
+    rows = [
+        {"id": 1, "image": b"updated"},
+        {"id": 2, "image": {"data": b"from-dict"}},
+        {"id": 3, "image": Blob.from_bytes(b"from-blob")},
+    ]
+
+    result = (
+        table.merge_insert("id")
+        .when_matched_update_all()
+        .when_not_matched_insert_all()
+        .execute(pd.DataFrame(rows) if as_pandas else rows)
+    )
+
+    assert result.num_updated_rows == 1
+    assert result.num_inserted_rows == 2
+    by_id = _row_ids_by_id(table)
+    assert table.fetch_blobs("image", [by_id[i] for i in range(1, 4)]).to_pylist() == [
+        b"updated",
+        b"from-dict",
+        b"from-blob",
+    ]
+
+
+def test_create_table_with_python_blob_dict_and_object():
+    db = lancedb.connect("memory:///")
+    schema = pa.schema([pa.field("id", pa.int64()), lancedb.blob("image")])
+    table = db.create_table(
+        "create_python_blobs",
+        data=[
+            {"id": 1, "image": {"data": b"from-dict"}},
+            {"id": 2, "image": Blob.from_bytes(b"from-blob")},
+        ],
+        schema=schema,
+    )
+
+    by_id = _row_ids_by_id(table)
+    assert table.fetch_blobs("image", [by_id[1], by_id[2]]).to_pylist() == [
+        b"from-dict",
+        b"from-blob",
+    ]