updateIndex on a mixed index fails an unrelated precondition when no key is in an applicable status

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- Version: `master` (`ac0eb23`)
- Storage Backend: any
- Mixed Index Backend: any
- Expected Behavior: `mgmt.updateIndex` on a mixed index whose keys are all already in the target state either no-ops or reports a clear error, as it does for composite indexes.
- Current Behavior: it fails an internal `Preconditions.checkArgument` with a message about `RelationTypeVertex` and composite indexes, which gives no indication of the real problem.

### Details

For a mixed index, `updateIndex` builds `keySubset` from the fields whose current status is in the action's applicable set:

https://github.com/JanusGraph/janusgraph/blob/ac0eb2392ddad3d96ab8cde2a5a9a123dbc5d839/janusgraph-core/src/main/java/org/janusgraph/graphdb/database/management/ManagementSystem.java#L1070-L1080

```java
keySubset = new HashSet<>();
MixedIndexType mixedIndexType = (MixedIndexType) indexType;
Set applicableStatus = updateAction.getApplicableStatus();
for (ParameterIndexField field : mixedIndexType.getFieldKeys()) {
if (applicableStatus.contains(field.getStatus()))
keySubset.add((PropertyKeyVertex) field.getFieldKey());
}
```

If no field matches, `keySubset` is empty. `setStatus` then routes to the vertex-level path:

https://github.com/JanusGraph/janusgraph/blob/ac0eb2392ddad3d96ab8cde2a5a9a123dbc5d839/janusgraph-core/src/main/java/org/janusgraph/graphdb/database/management/ManagementSystem.java#L1283-L1291

```java
private void setStatus(JanusGraphSchemaVertex vertex, SchemaStatus status, Set keys) {
if (keys.isEmpty()) setStatusVertex(vertex, status);
else setStatusEdges(vertex, status, keys);
...
}

private void setStatusVertex(JanusGraphSchemaVertex vertex, SchemaStatus status) {
Preconditions.checkArgument(vertex instanceof RelationTypeVertex || vertex.asIndexType().isCompositeIndex());
```

which is guaranteed to fail for a mixed index.

Note the asymmetry: the composite-index branch calls `updateAction.isApplicableStatus(schemaVertex.getStatus())` first, which either returns early or throws the helpful `"Update action [%s] cannot be invoked for index with status [%s]"`. The mixed-index branch performs no such validation, so it falls through into an unrelated precondition.

Realistic triggers, all of them idempotent schema scripts being re-run:

- `REGISTER_INDEX` after all keys are already `ENABLED` — applicable status is `{INSTALLED}`, so nothing matches.
- `DISABLE_INDEX` when no key is `ENABLED`/`DISABLED`/`REGISTERED`.
- `ENABLE_INDEX` on a freshly created index whose keys are all `INSTALLED`.

Separately, and possibly worth its own issue: because the mixed-index branch never calls `isApplicableStatus`, `DROP_INDEX` on a live mixed index is not gated on `DISCARDED` either. `schemaVertex.remove()` runs regardless, dropping the schema vertex while leaving every document in the index backend orphaned with no JanusGraph-side handle to remove them.

### Steps to Reproduce

```java
mgmt = graph.openManagement();
mgmt.buildIndex("byName", Vertex.class).addKey(name).buildMixedIndex("search");
mgmt.commit();
ManagementSystem.awaitGraphIndexStatus(graph, "byName").call();

mgmt = graph.openManagement();
mgmt.updateIndex(mgmt.getGraphIndex("byName"), SchemaAction.REGISTER_INDEX); // keys already ENABLED
// IllegalArgumentException from setStatusVertex
```

### Suggested Fix

Give the mixed-index branch the same guard the composite branch has: when `keySubset` is empty, either return `null` as the composite path does for an inapplicable status, or throw the descriptive `isApplicableStatus` error naming the action and the current field statuses.

Start in janusgraph-core/src/main/java/org/janusgraph/graphdb/database/management/ManagementSystem.java, reading updateIndex and the mixed-index path through setStatus. Reproduce the REGISTER_INDEX, DISABLE_INDEX, or ENABLE_INDEX cases with no applicable fields; done means the operation no-ops or reports the descriptive applicability error instead of the unrelated precondition failure.

Written by the indexing model from the issue text.