…on Count (elastic#143077)

The trained model stats API was computing `required_native_memory_bytes` incorrectly. Since elastic#98139, memory estimation has depended on the number of allocations, but the code used the total allocation count across all deployments instead of each deployment’s own count. As a result, when multiple NLP models were deployed with different allocation counts, every model’s `required_native_memory_bytes` was based on the same summed value, so changing allocations for one model incorrectly changed the reported memory for others. This only affected the Stats API output, not actual deployment behavior.

The fix computes `required_native_memory_bytes` per deployment using each deployment’s allocation count. `TransportGetTrainedModelsStatsAction` now passes `Map<String, AssignmentStats>` into `modelSizeStats()` instead of a summed allocation count, and `modelSizeStats()` emits per-deployment entries keyed by `deploymentId` with the correct `numberOfAllocations`. `GetTrainedModelsStatsAction.Response.Builder.build()` looks up model size stats by `deploymentId` first and falls back to `modelId` for undeployed or non-PyTorch models. Unit tests were added to cover per-deployment resolution, undeployed models, and the fallback path.


Fixes elastic#107831