Spring-ai Fix Mistral AI Chat model function call usage calculation

• Fix the chat model's call() to calculate the cumulative usage

• Use an explicit internalCall to pass the previous chat response so that accumulation can be done

• Fix the chat model's stream() to calculate the cumulative usage

• Fix MistralAi API to include usgae in ChatCompletionChunk
• Use internalStream() to accumulate the usage

Add/update tests

Comment From: tzolov

Thanks @ilayaperumalg !

Comment From: tzolov

rebased and merged at 81dfd3b6b5af1d17b4b5bafcbd7c9e2dd928c922