Default task failure cause to "workflow worker unhandled failure" #2705
Description
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Today, if my reading is right, we default RespondWorkflowTaskFailedRequest.cause to WORKFLOW_TASK_FAILED_CAUSE_UNSPECIFIED. This is unlike Go SDKs that defaults task failures to WORKFLOW_TASK_FAILED_CAUSE_WORKFLOW_WORKER_UNHANDLED_FAILURE. We should change to the latter in these cases where it is usually user exception (also see same issue in Core: temporalio/sdk-core#1041).