[BUG] Mistral: response format type lowercased with default locale before sending to the API

Problem

MistralAiResponseFormatType.toString() returns name().toLowerCase() using the default locale, and MistralAiResponseFormat.fromType() feeds that result into the request payload (type(type.toString())). Under a Turkish/Azeri default locale, any constant containing I (e.g. a future JSON_SCHEMA) would be sent with a dotless ı, producing an invalid value for the Mistral API.

Expected behavior

toString() lowercases with Locale.ROOT so the wire value is locale-independent (current constants TEXT/JSON_OBJECT are unaffected; this is a correctness fix for the wire path, not just hygiene).

Environment

langchain4j main (31950454f), module langchain4j-mistral-ai.