Add missing builder to follow project patterns

[jasonparallel]

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[dev-jonghoonpark]

🙌 Please fix the DCO check by signing your commit. git commit --amend --signoff @jasonparallel

[dev-jonghoonpark]

Please update license header (2024 → 2025)
(see: https://github.com/spring-projects/spring-ai/blob/main/CONTRIBUTING.adoc#update-license-header-to-modified-files-as-necessary)

/*
 * Copyright 2023-2025 the original author or authors.
 *

[dev-jonghoonpark]

Please update the existing codes(include test code) that uses constructor to use Builder.

[dev-jonghoonpark]

Could you please avoid changing the import order unless necessary? It helps maintain consistency.
Also, kindly avoid modifying the license header unless there's a specific reason.
Thank you!

[dev-jonghoonpark]

In my personal opinion, it would be better to handle the "Fix Formatter onn typos" task in a separate PR for files that are not related to the Mistral API.