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.

nodejs/lancedb/table.ts

@@ -225,9 +225,8 @@ export interface LsmWriteSpec {
   /** Bucket variant: the number of buckets, in `[1, 1024]`. */
   numBuckets?: number;
   /**
-   * Indexes the MemWAL keeps up to date. Omit to maintain every supported
-   * index, resolved on install — a snapshot, so indexes created later are not
-   * maintained. Pass `[]` for none.
+   * Indexes the MemWAL keeps up to date. Omit for every index the table has,
+   * including ones created later. Pass `[]` for none.
    */
   maintainedIndexes?: string[];
   /** Default `ShardWriter` configuration recorded in the MemWAL index. */
@@ -766,8 +765,8 @@ export abstract class Table {
    * ({@link Table#setUnenforcedPrimaryKey}); bucket sharding additionally
    * requires it to be the single column being bucketed.
    *
-   * Omitting `maintainedIndexes` maintains every index on the table, resolved
-   * here, failing if one cannot be maintained — name them to install anyway.
+   * Omitting `maintainedIndexes` maintains every index the table has,
+   * including ones created later, and skips a kind the MemWAL cannot maintain.
    * Naming them pins an exact set, and a still-building index is rejected
    * rather than quietly omitted.
    * @param {LsmWriteSpec} spec The sharding spec to install.
@@ -797,10 +796,9 @@ export abstract class Table {
    *
    * Resolves to `undefined` when the MemWAL LSM write path is not enabled (no
    * spec has been set, or it was removed with {@link Table#unsetLsmWriteSpec}).
-   * The returned spec mirrors what was passed to
-   * {@link Table#setLsmWriteSpec}, except that `maintainedIndexes` always
-   * reports the concrete list resolved when the spec was set — `undefined`
-   * never round-trips.
+   * The spec is the one installed, including its maintained-index selection:
+   * an absent `maintainedIndexes` for every index the table has, an empty
+   * array for none.
    * @returns {Promise<LsmWriteSpec | undefined>}
    */
   abstract getLsmWriteSpec(): Promise<LsmWriteSpec | undefined>;