fix: pin remote BlobFile reads to the opened version (#4295)

## Cause

On latest-tracking remote tables, `fetch_blob_files` probed a concrete
table version but retained `None` on each handle. Later range reads
resolved the latest version again, so an update or delete could make an
opened handle fail.

## Fix

For nonempty blobs, capture `x-lancedb-version` from the 206 size probe
and include it on every later range request. Reject a nonempty probe
without a valid version. Accept an empty-blob 416 probe with or without
that header, because an empty handle makes no later data range request.
The Python remote mock now covers both response shapes.

## Limitation

A table dropped and recreated with the same name can reuse version
numbers and row addresses. Version pinning does not identify the table
itself, so an old handle can still return data from the recreated table.
That identity case needs a separate fix. Callers that replace a table
should discard its old blob handles.

## Validation

- `cargo test --quiet --features remote -p lancedb --lib
remote::table::blobs::tests` (24 passed)
- Rebuilt the Python extension; `uv run --extra tests pytest
python/tests/test_remote_db.py -q -k remote_blob` (6 passed)
- `cargo clippy --quiet --features remote -p lancedb --lib --tests
--examples`
- `cargo fmt --all -- --check`, `ruff check .`, and `ruff format .`

Fixes #4276

<!-- lance-gatekeeper-fix:v1 agent=e309bdf75b1a87f6492a6dc3ee1bfd81
generation=1 -->

---------

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

python/python/tests/test_remote_db.py

@@ -2403,11 +2403,16 @@ def blob_remote_table(*, server_version=Version("0.5.0")):
         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]
+            payload = {10: b"alpha", 20: None, 30: b"gamma", 40: b""}[row_id]
             if payload is None:
                 request.send_response(204)
                 request.end_headers()
                 return
+            if not payload:
+                request.send_response(416)
+                request.send_header("Content-Range", "bytes */0")
+                request.end_headers()
+                return
             byte_range = request.headers["Range"].removeprefix("bytes=")
             start_text, end_text = byte_range.split("-", maxsplit=1)
             start = int(start_text)
@@ -2416,6 +2421,9 @@ def blob_remote_table(*, server_version=Version("0.5.0")):
             request.send_response(206)
             request.send_header("Content-Range", f"bytes {start}-{end}/{len(payload)}")
             request.send_header("Content-Length", str(len(chunk)))
+            request.send_header(
+                "x-lancedb-version", str(BLOB_DESCRIBE_RESPONSE["version"])
+            )
             request.end_headers()
             request.wfile.write(chunk)
         elif request.path == "/v1/table/test/query/":
@@ -2459,17 +2467,21 @@ def test_remote_blob_columns_and_fetch():
 
 def test_remote_blob_files_are_lazy_seekable_handles():
     with blob_remote_table() as table:
-        files = table.fetch_blob_files("image", [10, 20, 30])
+        files = table.fetch_blob_files("image", [10, 20, 30, 40])
 
-        assert len(files) == 3
-        alpha, null_row, gamma = files
+        assert len(files) == 4
+        alpha, null_row, gamma, empty = files
         assert null_row is None
         assert alpha is not None
         assert gamma is not None
+        assert empty is not None
         assert alpha.size() == 5
         assert alpha.read_range(1, 3) == b"lph"
         gamma.seek(2)
         assert gamma.read() == b"mma"
+        assert empty.size() == 0
+        assert empty.read() == b""
+        assert empty.read_range(0, 0) == b""
         alpha.close()
         assert alpha.closed
         with pytest.raises(RuntimeError, match="already closed"):