feat(remote): add seekable blob range reads (#3750)

## Summary

- Implements Cloud `fetch_blob_files`: returns real seekable `BlobFile`
handles over HTTP Range instead of `NotSupported`.
- Completes the second Cloud blob read verb after #3684 (`fetch_blobs` =
eager whole bytes; this = lazy / partial / sequential reads).
- Same public handle API as local (`read_range`, `read_up_to`, `seek`,
`tell`, `close`), so one code path works for local and Cloud.

Large blobs (video, audio, PDFs) should not require downloading the
whole object to inspect a header or stream a slice. After search,
callers open a handle and read only what they need:

```python
hits = table.search(vec).select(["id", "video"]).limit(5).to_arrow()

with table.fetch_blob_files("video", hits)[0] as f:
    header = f.read_range(0, 256)
    f.seek(keyframe_offset)
    chunk = f.read_up_to(1 << 20)
```

### Behavior

- Handle creation probes size with `bytes=0-0` (bounded concurrency,
input order preserved).
- `204` → null (`None`); `416` with `bytes */0` → valid empty blob;
other `416` → error.
- `read_range` validates `Content-Range` and body length; OOB ranges
fail with `invalid_input` before the request (aligned with Lance).
- `read_up_to` reuses one open-ended Range response across sequential
reads; `seek` drops it.
- Servers older than 0.5.0 get a clear `NotSupported` (does not suggest
`fetch_blobs`, which they also lack).

## Testing

- `cargo test --features remote -p lancedb remote_blob`
- `cargo test --features remote -p lancedb test_blob`
- `cargo clippy --features remote --tests --examples` (no new warnings
from this change)

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

python/python/lancedb/table.py

@@ -1595,8 +1595,8 @@ class Table(ABC):
         Prefer this over :meth:`fetch_blobs` for large payloads. ``row_ids`` is
         a ``list[int]`` or a query ``pyarrow.Table`` carrying row identity via
         ``_rowid`` or a ``_lance_row_id`` field on the blob descriptor. Null
-        rows are ``None``. Unsupported on LanceDB Cloud, where
-        :meth:`fetch_blobs` returns full bytes instead.
+        rows are ``None``. Remote tables require LanceDB Cloud server 0.5.0 or
+        newer.
         """
 
     @abstractmethod

python/python/tests/test_remote_db.py

@@ -2055,6 +2055,24 @@ def blob_remote_table(*, server_version=Version("0.5.0")):
             request.send_header("phalanx-version", str(server_version))
             request.end_headers()
             request.wfile.write(json.dumps(BLOB_DESCRIBE_RESPONSE).encode())
+        elif request.path.startswith("/v1/table/test/blob/image/"):
+            path = request.path.partition("?")[0]
+            row_id = int(path.split("/")[-2])
+            payload = {10: b"alpha", 20: None, 30: b"gamma"}[row_id]
+            if payload is None:
+                request.send_response(204)
+                request.end_headers()
+                return
+            byte_range = request.headers["Range"].removeprefix("bytes=")
+            start_text, end_text = byte_range.split("-", maxsplit=1)
+            start = int(start_text)
+            end = int(end_text) if end_text else len(payload) - 1
+            chunk = payload[start : end + 1]
+            request.send_response(206)
+            request.send_header("Content-Range", f"bytes {start}-{end}/{len(payload)}")
+            request.send_header("Content-Length", str(len(chunk)))
+            request.end_headers()
+            request.wfile.write(chunk)
         elif request.path == "/v1/table/test/query/":
             content_len = int(request.headers.get("Content-Length", 0))
             body = json.loads(request.rfile.read(content_len))
@@ -2092,8 +2110,21 @@ def test_remote_blob_columns_and_fetch():
         assert table.blob_columns() == ["image"]
         blobs = table.fetch_blobs("image", [10, 20, 30])
         assert blobs.to_pylist() == [b"alpha", None, b"gamma"]
-        with pytest.raises(NotImplementedError, match="Use fetch_blobs for full bytes"):
-            table.fetch_blob_files("image", [10, 20, 30])
+
+
+def test_remote_blob_files_are_lazy_seekable_handles():
+    with blob_remote_table() as table:
+        files = table.fetch_blob_files("image", [10, 20, 30])
+
+        assert len(files) == 3
+        alpha, null_row, gamma = files
+        assert null_row is None
+        assert alpha is not None
+        assert gamma is not None
+        assert alpha.size() == 5
+        assert alpha.read_range(1, 3) == b"lph"
+        gamma.seek(2)
+        assert gamma.read() == b"mma"
 
 
 def test_remote_blob_fetch_accepts_query_table():