[Backport 8.19] Update Inference specification for Mistral's completion and chat completion tasks

[Jan-Kazlouski-elastic]

backport 42b4977 from #4464

Removed intially included changes for TaskType.ts from @kosabogi 's #4444 because they are a part of the main branch and were already backported to 8.18.

FYI @kosabogi @jonathan-buttner

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
inference.chat_completion_unified	⚪	Missing test	Missing test
inference.completion	⚪	Missing test	Missing test
inference.delete	⚪	Missing test	Missing test
inference.get	🟢	1/1	1/1
inference.inference	⚪	Missing test	Missing test
inference.put_alibabacloud	⚪	Missing test	Missing test
inference.put_amazonbedrock	⚪	Missing test	Missing test
inference.put_anthropic	⚪	Missing test	Missing test
inference.put_azureaistudio	⚪	Missing test	Missing test
inference.put_azureopenai	⚪	Missing test	Missing test
inference.put_cohere	⚪	Missing test	Missing test
inference.put_elasticsearch	⚪	Missing test	Missing test
inference.put_elser	⚪	Missing test	Missing test
inference.put_googleaistudio	⚪	Missing test	Missing test
inference.put_googlevertexai	⚪	Missing test	Missing test
inference.put_hugging_face	⚪	Missing test	Missing test
inference.put_jinaai	⚪	Missing test	Missing test
inference.put_mistral	⚪	Missing test	Missing test
inference.put_openai	⚪	Missing test	Missing test
inference.put_voyageai	⚪	Missing test	Missing test
inference.put_watsonx	⚪	Missing test	Missing test
inference.put	⚪	Missing test	Missing test
inference.rerank	⚪	Missing test	Missing test
inference.sparse_embedding	⚪	Missing test	Missing test
inference.stream_completion	⚪	Missing test	Missing test
inference.text_embedding	⚪	Missing test	Missing test
inference.update	⚪	Missing test	Missing test

You can validate these APIs yourself by using the make validate target.