[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

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