[Bug]: Update TaskState enum UNRECOGNIZED to TASK_STATE_UNSPECIFIED

[wushunhao]

What happened?

Hi, I am looking at v1.1.0.Final version.

I found that the latest version of the org.a2aproject.sdk.spec.TaskState enum class still has value UNRECOGNIZED instead of TASK_STATE_UNSPECIFIED, which is inconsistent with the a2a-protocol.

Please fix this so that we don't have to do the conversion and validation ourselves.

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