Validate the input must be a struct when creating tools

[git-hulk]

This may partially resolve #330 by checking if the input is a struct type.

[gemini-code-assist]

Summary of Changes

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This pull request enhances the functiontool package by implementing a crucial validation step during tool creation. It now mandates that all input arguments for a function tool must be defined as a struct, thereby standardizing the input schema and preventing potential issues with unstructured data types. This modification improves the reliability of the tool creation process and is supported by comprehensive test updates and additions.

Highlights

• Input Type Validation: Introduced a new validation check in functiontool.New to ensure that the input arguments (TArgs) for creating a tool must be a struct type. This prevents the use of primitive types directly as tool inputs.
• Issue Resolution: This change partially addresses issue [Bug] llmagent is only compatible with functiontool where input is a struct #330, improving the robustness of tool creation by enforcing a structured input format.
• Test Updates: Existing test cases in function_test.go were updated to conform to the new validation rule, converting primitive input types to wrapper struct types. New test cases were added to specifically verify that functiontool.New correctly rejects non-struct input types.

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[gemini-code-assist]

Code Review

This pull request adds a validation check to ensure that function tool inputs are structs. The reflection-based implementation in function.go is solid and correctly handles pointers. The tests are also well-updated, with new cases in TestNew_InvalidInputType to cover the new validation.

I found a minor copy-paste error in a new test case that would cause a compilation failure, and I've provided a suggestion to fix it.

Additionally, it appears tool/tool_test.go was not updated and contains a test that will now fail. You'll likely need to update it to use a struct-based tool to get the build to pass.