Improve Mistral AI Chat documentation for reasoning models and ReasoningEffort Javadoc

[nicolaskrier]

Description

• Clarify adjustable versus native reasoning model types,
• Expand on metadata use,
• Use constant instead of string for thinking content metadata,
• Complement ReasoningEffort Javadoc,
• Add latest tag to Mistral Small model within an application.properties snippet.

Explanations

• I have discovered that Mistral AI's reasoning models are well explained within Mistral AI Studio API documentation.
• reference_content metadata is not relative to reasoning models (cf MistralAiChatModelIT.referenceCall).
• mistral-small seems to be the old version of Mistral Small. I have updated it to the latest tag due to this error:

curl -s https://api.mistral.ai/v1/models/mistral-small \       
  -H "Authorization: Bearer $MISTRAL_AI_API_KEY"  
{"object":"error","message":"Model mistral-small is deprecated.","type":"invalid_model","param":null,"code":"1500","raw_status_code":404}

Proposed milestone

2.0.0-RC1

Related work

• Improve MistralAiApi Jackson mapping for message content #5585
• Add reasoning effort configuration property