[ML] Increase Test Coverage 8.17.0

[peteharverson]

Follow up to #188459

Meta issue to summarize improvements to test coverage for 8.17.0.

Jest unit tests

• [ML] AIOps: Adds Log Rate Analysis embeddable for dashboards #197943 (WR)
• [ML] Transforms: Improve percentiles agg validation #197816 (RJ)
• [ML] Trained Model: Fix start deployment with ML autoscaling and 0 active nodes  #201256 (DA)

API integration tests

• POST /internal/ml/anomaly_detectors/_validate/detector (JG) (remove endpoint) [ML] Removing validate detector endpoint #198667

Serverless tests

Functional tests

• Memory usage page (include check for memory usage chart) (RJ) [ML] Memory usage: Functional tests #197415
• Log rate analysis embeddable (WR) [ML] AIOps: Adds Log Rate Analysis embeddable for dashboards #197943
• Log rate analysis - search bar edit (MA) [ML][AIOPS] Log rate analysis: add basic functional test  #199054

Performance testing journey

Accessibility tests

part of #88496

• Memory usage page (RJ) [ML] Memory usage accessibility tests #198306

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

Closing, all items planned for 8.17 have been completed.