feat(remote): support add_columns with schema in remote client (#4244)

Support adding column with pyarrow schema in remote client. Previously
this raised an error. This achieves parity with local client.
Server-side implementation is already complete.

`RemoteTable::add_columns` matched only `SqlExpressions` and refused
everything else, so `add_columns(pa.Field | List[pa.Field] | pa.Schema)`
reached a remote table as `NotSupported`. Every layer above was already
in place: the Python and Node surfaces accept a schema, both bindings
build `NewColumnTransform::AllNulls` from it, and the Function-binding
guard already reads that variant's column names. Only the arm that turns
it into a request was missing.

Send the schema as an Arrow IPC schema message under
`application/vnd.apache.arrow.stream`, which is what the server takes.
It is also the only encoding that round-trips a field whole: the JSON
type representations either hide decimal precision in a length field or
drop a timestamp's unit and timezone. With no JSON envelope the branch
rides the query string, as it does for the other binary-bodied
endpoints.

The response handling is now shared by both arms rather than living
inside the SQL one, so the new path gets the same schema-cache
invalidation, write-version tracking, and old-server empty-body
fallback.

Also widen the sync `RemoteTable.add_columns` annotation to match
`Table.add_columns`. It delegates to the async table, so a schema
already worked at runtime; only the signature and its docs disagreed.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

python/python/lancedb/remote/table.py

@@ -982,6 +982,9 @@ class RemoteTable(Table):
         self,
         transforms: Dict[str, str | FunctionApplication]
         | FunctionApplication
+        | pa.Field
+        | List[pa.Field]
+        | pa.Schema
         | None = None,
         *,
         computed: Dict[str, str] | None = None,