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.

docs/src/js/classes/Table.md

@@ -662,10 +662,9 @@ Read the [LsmWriteSpec](../interfaces/LsmWriteSpec.md) currently installed on th
 
 Resolves to `undefined` when the MemWAL LSM write path is not enabled (no
 spec has been set, or it was removed with [Table#unsetLsmWriteSpec](Table.md#unsetlsmwritespec)).
-The returned spec mirrors what was passed to
-[Table#setLsmWriteSpec](Table.md#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
 
@@ -1107,8 +1106,8 @@ All variants require the table to have an unenforced primary key
 ([Table#setUnenforcedPrimaryKey](Table.md#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.
 

docs/src/js/interfaces/LsmWriteSpec.md

@@ -34,9 +34,8 @@ Bucket and identity variants: the sharding column.
 optional maintainedIndexes: string[];
 ```
 
-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.
 
 ***