[Feat]: Test for blocking vs non-blocking sendMessage requests

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

_No response_

### Describe the solution you'd like

Until https://github.com/a2aproject/a2a-java/pull/327 we ignored blocking vs non-blocking behaviour, but still passed the TCK. So it seems the TCK needs to test for this.

### 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

Review pull request #327 in the a2a-java repository to understand the blocking vs non-blocking behavior. Examine the existing TCK tests to identify where this behavior should be validated. Implement new test cases in the TCK to verify correct handling of sendMessage requests, ensuring they pass with the updated implementation.

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