[Feat]: netstandard 2.0 support

[lofcz]

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

Thanks for maintaining this project, we are using it in 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

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