[ZEPPELIN-5876] Implement DockerInterpreterProcess.isAlive() using container state

[hyunw9]

What is this PR for?

The Docker launcher's isAlive() isn't really implemented. It just calls isRunning(), which only checks whether the interpreter's Thrift port accepts a connection (checkIfRemoteEndpointAccessible). That tells you the port is open, not that the process behind it is alive. A container can be OOMKilled or already gone and still look alive if the port happens to answer for a moment. It also doesn't match the InterpreterClient contract, which says isAlive should reflect process status and stay separate from running. The method still had //TODO(ZEPPELIN-5876): Implement it more accurately sitting on it.

While I was there I filled in getErrorMessage(), which always returned null. When the container isn't running it now says why: OOMKilled, or a non-zero exit code. That logic lives in a small describeContainerFailure() helper to keep getErrorMessage() short.

One change is test-only: the docker field is now 'VisibleForTesting', so tests can inject a mock DockerClient and skip needing a real daemon.

What type of PR is it?

Feature

Todos

• Implement isAlive() from the container's actual state (running/paused)
• Implement getErrorMessage() (OOMKilled / exit code)
• Unit tests with a mocked DockerClient

What is the Jira issue?

[ZEPPELIN-5876]

How should this be tested?

Automated unit tests in DockerInterpreterProcessTest :

• isAlive_trueWhenContainerRunning - running == true → alive
• isAlive_falseWhenContainerExitedOrOomKilled — running == false → not alive
• getErrorMessage_reportsOomKilled - oomKilled == true → message contains the reason

Screenshots (if appropriate)

Questions:

• Does the license files need to update? - no
• Is there breaking changes for older versions? - no
• Does this needs documentation? - no