[ML] Add support for returning pt_tiny_elser from the ml.trainedModelsProvider.getELSER() helper function

[spong]

In plumbing through the Security Assistant Knowledge Base API integration tests (#192665), I ended up having to add support for a modelId override to our API's when interacting with the Knowledge Base as we currently use the ml.trainedModelsProvider.getELSER() helper (which does not return pt_tiny_elser in test environments) to get the modelId for setting up ELSER, ingest pipelines, retrievers, etc. The O11y Assistant folks also have an open issue (#192757) to do the same.

It would be nice if the getELSER() helper function below could either be overridden to return pt_tiny_elser in our test environments, or if our test environments could be updated such that the function returns pt_tiny_elser.

kibana/x-pack/plugins/ml/server/models/model_management/models_provider.ts

Lines 542 to 552 in 68a9244

	/**
	* Provides an ELSER model name and configuration for download based on the current cluster architecture.
	* The current default version is 2. If running on Cloud it returns the Linux x86_64 optimized version.
	* If any of the ML nodes run a different OS rather than Linux, or the CPU architecture isn't x86_64,
	* a portable version of the model is returned.
	*/
	async getELSER(
	options?: GetModelDownloadConfigOptions
	): Promise<ModelDefinitionResponse> | never {
	return await this.getCuratedModelConfig('elser', options);
	}

Note

Perhaps the need for this will go away or change somewhat when we switch from managing ELSER via the trainedModels API to using a default inference endpoint (#192461).