[Zhoujie628]

Closes #58

Summary

• Rename the starter artifact to spring-boot-a2a-starter across the POM, samples dependency, README and bilingual docs
• Increase the default Notification-T acknowledgement timeout from 5 seconds to 5 minutes; expose the sample Spring setting as a2a.notification-ack-timeout-seconds / A2A_NOTIFICATION_ACK_TIMEOUT_SECONDS
• Ensure the simulator keeps Notification-T SSE streams open (10-minute safety net) so the demo cleanup closes them cleanly as SEND_CANCELLED instead of the simulator idle timeout surfacing as SEND_FAILED
• Align WorkbenchExtensionLifecycle to wait for the notification ack within the configurable budget and propagate cancellation

Test plan

• Full main reactor mvn -B clean verify green
• SpringSpnDemoOrderE2ETest: SEND_FAILED=0, SEND_CANCELLED=8 (clean thread-interrupt close)
• SpringSpnDemoE2ETest: 4/4 green, no idle-timeout errors