fix: read back the maintained-index selection a table was given (#4447)

`maintained_indexes` on a MemWAL table has three values: a named list,
`[]` for none, and `null` for every index the table has, including ones
created later. This PR makes every client read all three back as they
were set.

- **Rust remote client** (also used by the Python and Node SDKs) reads
`null` as `None`.
- **Java client** reads `null` as `null`, not as an empty list.
- **Docs** for the setter, the field and the getter describe the three
values the same way in Rust, Python, Node and Java. The generated JS
docs are regenerated to match.

## Testing

- `rust/lancedb`: 73 LSM tests pass, including one case per value. With
the decoder typed as a list, the `null` case fails.
- Java: two cases for `null` and `[]` on the read path, run by the Java
CI job.
- `cargo fmt`, `cargo clippy -D warnings` and `ruff format` are clean.

python/python/lancedb/table.py

@@ -5445,11 +5445,16 @@ class AsyncTable:
     async def get_lsm_write_spec(self) -> Optional["LsmWriteSpec"]:
         """Read the LsmWriteSpec currently installed on this table.
 
-        Returns ``None`` when the MemWAL LSM write path is not enabled (no
-        spec has been set, or it was removed with `unset_lsm_write_spec`).
-        The returned spec mirrors what was passed to `set_lsm_write_spec`,
-        except that ``maintained_indexes`` always reports the concrete list
-        resolved when the spec was set — ``None`` never round-trips.
+        Returns ``None`` when the LSM write path is not enabled at all — no
+        spec has been set, or one was removed with `unset_lsm_write_spec`.
+        That is a different answer from a spec whose ``maintained_indexes``
+        is ``None``, which is an installed spec selecting indexes
+        automatically.
+
+        The spec read back is the one that was installed, selection
+        included: ``None`` maintains every supported index the table has now
+        or gains later, ``[]`` maintains none, and a non-empty list maintains
+        exactly those. All three round-trip.
         """
         return await self._inner.get_lsm_write_spec()