add unit test coverage for LinkExecution

[aglinxinyuan]

Task Summary

LinkExecution (amber/src/main/scala/org/apache/texera/amber/engine/architecture/controller/execution/LinkExecution.scala) tracks per-channel execution state for a physical link but currently has no unit tests. Add LinkExecutionSpec covering:

• A fresh LinkExecution exposes no channel executions
• initChannelExecution registers a new ChannelExecution for the given channel id
• A second initChannelExecution for the same channel id raises an AssertionError
• Multiple distinct channel ids are tracked independently

Priority

P3 – Low

Task Type

• Testing / QA