Add CI leg for running A2A TCK tests

Currently, we run the tests manually using the instructions provided [here](https://github.com/a2aproject/a2a-dotnet/tree/main/samples/AgentServer#running-the-tck). I broke them at least once, I saw others do it to (by accident).

As suggested by @Blackhex, it would be great to automate this process.

The issue references running A2A TCK tests manually via instructions in the AgentServer sample. Start by examining the existing GitHub Actions workflow files in .github/workflows to understand the CI setup. Review the linked instructions to understand the test commands and dependencies. The goal is to add a new CI job that automates these steps, likely using Docker or a specific runner configuration.

Escrito por el modelo de indexación a partir del texto del issue.