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.

java/lancedb-core/src/test/java/com/lancedb/LanceDbTableLsmTest.java

@@ -215,6 +215,61 @@ public class LanceDbTableLsmTest {
     assertEquals("true", spec.get().writerConfigDefaults().get("durable_write"));
   }
 
+  /** A null selection, every index the table has, reads back as null rather than an empty list. */
+  @Test
+  public void testGetLsmWriteSpecRoundTripsEveryIndexSelection() {
+    enqueue(
+        "get_lsm_write_spec",
+        200,
+        "{\"lsm_write_spec\":{\"sharding\":{\"mode\":\"unsharded\"},"
+            + "\"maintained_indexes\":null,\"writer_config_defaults\":{}}}");
+
+    Optional<LsmWriteSpec> spec = lsm.getLsmWriteSpec();
+
+    assertTrue(spec.isPresent());
+    assertNull(spec.get().maintainedIndexes());
+  }
+
+  @Test
+  public void testGetLsmWriteSpecRoundTripsNoIndexSelection() {
+    enqueue(
+        "get_lsm_write_spec",
+        200,
+        "{\"lsm_write_spec\":{\"sharding\":{\"mode\":\"unsharded\"},"
+            + "\"maintained_indexes\":[],\"writer_config_defaults\":{}}}");
+
+    Optional<LsmWriteSpec> spec = lsm.getLsmWriteSpec();
+
+    assertTrue(spec.isPresent());
+    assertEquals(Collections.emptyList(), spec.get().maintainedIndexes());
+  }
+
+  /**
+   * Null is the automatic selection, so anything that is neither null nor an array has to be
+   * rejected: reading a string or an object as null would turn a malformed response into a policy.
+   */
+  @Test
+  public void testGetLsmWriteSpecRejectsANonArrayMaintainedIndexes() {
+    enqueue(
+        "get_lsm_write_spec",
+        200,
+        "{\"lsm_write_spec\":{\"sharding\":{\"mode\":\"unsharded\"},"
+            + "\"maintained_indexes\":\"id_idx\",\"writer_config_defaults\":{}}}");
+
+    assertThrows(IllegalStateException.class, () -> lsm.getLsmWriteSpec());
+  }
+
+  @Test
+  public void testGetLsmWriteSpecRejectsNonStringIndexNames() {
+    enqueue(
+        "get_lsm_write_spec",
+        200,
+        "{\"lsm_write_spec\":{\"sharding\":{\"mode\":\"unsharded\"},"
+            + "\"maintained_indexes\":[7],\"writer_config_defaults\":{}}}");
+
+    assertThrows(IllegalStateException.class, () -> lsm.getLsmWriteSpec());
+  }
+
   @Test
   public void testGetLsmWriteSpecAbsent() {
     enqueue("get_lsm_write_spec", 200, "{\"lsm_write_spec\":null}");