[ML] Increase Test Coverage 8.3.0

[peteharverson]

Follow up to #125095.

Meta issue to summarize improvements to test coverage for 8.3.0.

API integration tests

• GET /api/ml/jobs/groups (JG) ([ML] Tests for get groups endpoint #133625)
• POST /api/ml/results/partition_fields_values (DA) ([ML] API integration tests for partition_fields_values endpoint #133519)
• POST /api/ml/results/max_anomaly_score (DA) ([ML ]API integration tests for max_anomaly_score endpoint #133776)
• POST /internal/aiops/example_stream — tests that show usage with streaming or bulk option (WR)
  [ML] Explain log rate spikes: Plugin setup #131317

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

Closing issue as work for 8.3.0 has been completed.