feat: add get_lsm_write_spec to read the installed LSM write spec (#3631)

## Summary

Adds `Table::get_lsm_write_spec` returning `Option<LsmWriteSpec>` — the
read counterpart to the existing `set_lsm_write_spec` /
`unset_lsm_write_spec`. Returns `None` when the MemWAL LSM write path is
not enabled; otherwise reconstructs the spec (mode, shard column,
`num_buckets`, `maintained_indexes`, `writer_config_defaults`) exactly
as installed.

## Changes

- **Rust core (`NativeTable`)** — reconstructs the spec from
`mem_wal_index_details()`, resolving the shard column from its Lance
field id via the dataset schema. This is a raw metadata read, so it is
unaffected by `describe_indices` system-index filtering.
- **Remote (`RemoteTable`)** — reads the `__lance_mem_wal` system index
through `index/list` with `include_system: true` (so the curated
`list_indices` surface stays unchanged), then parses the index `details`
JSON. It matches the index by name and ignores `index_type`, so no
client `IndexType` variant is needed. It uses the **server-resolved
`column` name** from the details (Lance field ids do not travel to the
remote client).
- **Python + TypeScript bindings** — sync and async, mirroring
`set`/`unset`, with round-trip tests (bucket / identity / unsharded,
plus `None` when unset).

## Tests

- Rust: native round-trip unit test + remote mock-endpoint tests
(present + absent). All green (`cargo test --features remote -p
lancedb`).
- Python/TS: round-trip tests added; binding-runtime execution runs in
CI.

## Dependencies for the remote path

The remote path is complete on the client side but depends on two
out-of-repo pieces to work end-to-end:
1. **lance** — emit the server-resolved shard **`column`** name in the
MemWAL index `details` JSON (field ids can't reach the client). See
lance-format/lance#7667.
2. **server** — honor `include_system` on `index/list` so the
`__lance_mem_wal` entry is returned for this read.

Against an older server (no `include_system`), the remote getter
degrades gracefully to `Ok(None)` rather than erroring.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

nodejs/lancedb/table.ts

@@ -585,6 +585,17 @@ export abstract class Table {
    * @returns {Promise<void>}
    */
   abstract unsetLsmWriteSpec(): Promise<void>;
+  /**
+   * Read the {@link LsmWriteSpec} currently installed on this 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 — including its `maintainedIndexes` and
+   * `writerConfigDefaults` — mirrors what was passed to
+   * {@link Table#setLsmWriteSpec}.
+   * @returns {Promise<LsmWriteSpec | undefined>}
+   */
+  abstract getLsmWriteSpec(): Promise<LsmWriteSpec | undefined>;
   /**
    * Drain and close any cached MemWAL shard writers held for this table.
    *
@@ -1091,6 +1102,15 @@ export class LocalTable extends Table {
     return await this.inner.unsetLsmWriteSpec();
   }
 
+  async getLsmWriteSpec(): Promise<LsmWriteSpec | undefined> {
+    // The native binding types `specType` as a plain `string`; narrow it back
+    // to the public union. The Rust `From` impl only ever emits one of the
+    // three valid values, so the cast is safe.
+    return ((await this.inner.getLsmWriteSpec()) ?? undefined) as
+      | LsmWriteSpec
+      | undefined;
+  }
+
   async closeLsmWriters(): Promise<void> {
     return await this.inner.closeLsmWriters();
   }