[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