refactor: fold the maintained-index API down to one entry point

Three cleanups to the LsmWriteSpec surface, all removing something:

`with_no_maintained_indexes` is gone. `with_maintained_indexes` now takes
`impl Into<Option<Vec<String>>>`, so `None` asks for every supported index,
a list pins an exact set, and an empty list maintains none — one method
covering what two did, and the argument reads the same as the field it sets.

`IndexConfig::is_memwal_maintainable` is gone. What a memtable can build is
lance's to say, and lance already says it via `is_maintainable_index_type`;
mirroring that onto a lancedb type only added a place for the two to drift.

`resolve_maintained_indexes` is now public and takes `&[IndexConfig]` rather
than a table. It was already the whole of the resolution rules — infer from
the committed set, or validate a named one — and a server installing specs
on a caller's behalf needs exactly those rules. Sharing it means sophon no
longer keeps a second copy that could disagree about what an absent list
means, and the error now names the table's indexes so a rejected call says
what was actually available.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

nodejs/src/table.rs

@@ -805,13 +805,11 @@ impl TryFrom<LsmWriteSpec> for lancedb::table::LsmWriteSpec {
                 )));
             }
         };
-        // An omitted `maintainedIndexes` keeps the spec's default of "every
+        // An omitted `maintainedIndexes` is the spec's default of "every
         // index the MemWAL can maintain".
-        let spec = match value.maintained_indexes {
-            Some(names) => spec.with_maintained_indexes(names),
-            None => spec,
-        };
-        Ok(spec.with_writer_config_defaults(writer_config_defaults))
+        Ok(spec
+            .with_maintained_indexes(value.maintained_indexes)
+            .with_writer_config_defaults(writer_config_defaults))
     }
 }