fix(splits): Make split assignment idempotent

[ArcticFaded]

[!NOTE] Replaces add/remove split mutations with a single idempotent set-example-splits operation and updates the UI to use it with improved menu behavior.

• GraphQL API:
  • Remove addDatasetExamplesToDatasetSplits and removeDatasetExamplesFromDatasetSplits inputs/mutations/payloads.
  • Add setDatasetExampleSplits with SetDatasetExampleSplitsInput and SetDatasetExampleSplitsMutationPayload returning a single example.
• Backend (src/phoenix/server/api/mutations/dataset_split_mutations.py):
  • Implement set_dataset_example_splits to atomically set splits for an example (validates IDs, deletes missing, inserts new; uses joinedload and tuple_).
  • Remove legacy add/remove example-split mutations and related types.
• Frontend (app/src/pages/examples/ExamplesSplitMenu.tsx):
  • Switch to ExamplesSplitMenuSetDatasetExampleSplitsMutation and update apply logic to send full splitIds per exampleId.
  • Change apply menu to single-select with manual open-state control to keep menu open on actions; add onAction handler and checkbox state sync.
  • Remove generated file for old remove mutation; add/update generated file for new set mutation.

^{Written by Cursor Bugbot for commit 18bb22d2901a75950eebc305c6a108b2d9f6ebea. This will update automatically on new commits. Configure here.}