[Feat]: netstandard 2.0 support

### Is your feature request related to a problem? Please describe.

Thanks for maintaining this project, we are using it in [LlmTornado](https://github.com/lofcz/LlmTornado) for our A2A adapter. Our library supports netstandard 2.0, as many users are targeting different platforms than .net 8/9/10. Currently `LlmTornado.A2A` is forced to support only the modern .net core versions due to this.

### Describe the solution you'd like

netstandard 2.0 support

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Check the project's target frameworks in the .csproj files. Understand what netstandard 2.0 compatibility entails for the existing codebase, including dependencies and API usage. Look for any conditional compilation or multi-targeting setup. Verify the build and test process after adjustments.

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