[ML] Prevent get datafeeds stats API returning an error when local tasks are slow to stop

[aishwarysingh7754]

To solve this issue, I modified the fromResponses method in the Response class to handle duplicate keys more gracefully. The solution involved:

Grouping the running states by datafeedId.
Sorting the running states within each group based on creation time in descending order.
Keeping only the state for the most recent task (the first element after sorting) for each datafeedId.

Fixes #104160

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[droberts195]

Thanks for the contribution @aishwarysingh7754.

Please could you push an extra file to your branch, called docs/changelog/104352.yaml and containing this:

pr: 104352
summary: Prevent get datafeeds stats API returning an error when local tasks are slow to stop
area: Machine Learning
type: bug
issues:
 - 104160

Also, you'll find that our automatic code formatter won't like the formatting changes you've made. We use an automatic code formatter because although nobody 100% likes the way it lays out the code it avoids all arguments about the best layout and avoids needless churn from people changing the layout.

To auto-format this change please run ./gradlew :x-pack:plugin:ml:spotlessApply from the top level of the repo. Then you'll be able to see what it changed and commit the changed file.