StreamJsonRpc use is being surfaced via exceptions

[stephentoub]

StreamJsonRpc is supposed to be an implementation detail of the .NET implementation. But its use is surfacing via StreamJsonRpc.RemoteInvocationException exceptions that emerge, and these are being documented and tested as part of its contract. Such exceptions should be caught and replaced either by an IOException, or if it's really important to call these out as JSON-RPC issues, a custom GH copilot exception type that surfaces the relevant information.

cc: @SteveSandersonMS

[github-actions]

Thanks for opening this issue, @stephentoub!

I've labeled this as sdk/dotnet and enhancement. You're right that StreamJsonRpc implementation details shouldn't leak through the public API surface. Wrapping these in either IOException or a custom Copilot-specific exception type would provide better abstraction and API stability.

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