fix: Use JsonBaseModel in FunctionTool (re. #473)

[vorburger]

Related to #473.

Intentionally just for FunctionTool, for now; but this should eventually really be done everywhere like this (IMHO).

#487 is a possible future follow-up, extending this with more flexibility; before working further on that, I want to see if this will already get LGTM, as is; and how long it takes to get this reviewed and merged... 🤣 I would pick that (#487) up later.

[gemini-code-assist]

Summary of Changes

Hello @vorburger, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request standardizes the JSON serialization and deserialization process within the FunctionTool by integrating JsonBaseModel. This change aims to improve code consistency and maintainability by centralizing the ObjectMapper configuration, aligning FunctionTool with a broader architectural pattern for managing JSON operations.

Highlights

• Refactored JSON Mapping: The FunctionTool has been updated to utilize JsonBaseModel for its ObjectMapper instance, centralizing JSON serialization/deserialization configuration.
• Dependency Streamlining: The direct dependency on Jdk8Module within FunctionTool has been removed, as its functionality is now handled through JsonBaseModel.
• Consistent ObjectMapper Usage: The OBJECT_MAPPER in FunctionTool is now initialized by calling JsonBaseModel.getMapper(), promoting a consistent approach to JSON processing across the codebase.

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[gemini-code-assist]

Code Review

This pull request refactors FunctionTool to use a shared ObjectMapper from JsonBaseModel, which is a good move for consistency. However, the implementation changes OBJECT_MAPPER to an instance field from a static one. This introduces a critical issue, as the field is accessed by static methods, which will lead to a compilation failure. My review includes a code suggestion to correct this by reverting it to a static final field and placing it with other static members for better organization.