Collectors: Add predefined OS filter options to instances table

## Context

Split from Graylog2/graylog-plugin-enterprise#13652 (sub-issue 3, remaining polish).

OS filtering on the instances table already works — the `os` attribute is marked `filterable(true)` in `CollectorInstancesResource.java`. However, users must type freeform values because there are no predefined `filterOptions`.

## Suggested Fix

Add `filterOptions` to the `os` `EntityAttribute` definition, similar to how `status` offers "Online"/"Offline":

```java
.filterOptions(Set.of(
FilterOption.create("linux", "Linux"),
FilterOption.create("windows", "Windows"),
FilterOption.create("darwin", "macOS")))
```

This gives users a dropdown instead of requiring them to know the exact string values.

## Relevant Files

- `graylog2-server/src/main/java/org/graylog/collectors/rest/CollectorInstancesResource.java` (line ~147)