Fix Gemini Tool Calling for texts returned from MethodToolCallback

[nlinhvu]

MethodToolCallback can return a String(which is Textual Json/Json String) that is not supported by jsonToStruct method. => Update jsonToStruct method with Textual Json/Json String.

MethodToolCallback relies on DefaultToolCallResultConverter which is not always return Textual Json/Json String, in case of methodReturnType is void, it simply returns simple String: "Done" that cannot be parsed by jsonToStruct => Update DefaultToolCallResultConverter always return Textual Json/Json String.

[tzolov]

@nlinhvu thank you for catching this

[tzolov]

Rebased and merged at f2df87b

[cho-thinkfree-com]

The issue I found is slightly different from this one.

When creating an exception processor like this:

DefaultToolExecutionExceptionProcessor(false)

The exception message is passed into the ToolResponse as a plain String in the responseData field.
Since it is delivered as a plain string rather than a JSON string, it causes errors when processed by the Vertex AI Chat Model.

I have submitted a fix for this in PR #4521