[ZEPPELIN-6574] Add read-only REST API for interpreter process status

[hyunw9]

What is this PR for?

Zeppelin server owns interpreter processes but exposes no API to see which ones are running. This adds a read-only snapshot endpoint:

GET /api/interpreter/status

[{
  "settingId": "spark", "settingName": "spark", "groupId": "spark-shared_process",
  "numSessions": 2, "started": true, "host": "127.0.0.1", "port": 51037,
  "startTime": "2026-08-09 14:20:11", "uptimeSeconds": 3184, "errorMessage": null
}]

Review point:

• No remote probe. Built from in-memory server state only, so a stuck interpreter cannot block the call. started means a process handle exists, not that the process is reachable. A bounded liveness probe is deliberately left to ZEPPELIN-6576.

First sub-task of ZEPPELIN-6568 (server-side status and idle lifecycle for interpreter processes); the remaining sub-tasks build on this.

What type of PR is it?

Feature

Todos

• - Add InterpreterProcessStatus, a snapshot DTO built from a ManagedInterpreterGroup
• - Add InterpreterSettingManager#getInterpreterProcessStatuses to aggregate all running groups
• - Add RemoteInterpreterProcess#getStartTimeMs so uptime is computed without a remote call
• - Add the GET /api/interpreter/status endpoint
• - Add unit and REST tests

What is the Jira issue?

• ZEPPELIN-6574

How should this be tested?

Included tests, both passing on current master:

• InterpreterSettingManagerTest#testGetInterpreterProcessStatuses - aggregation is empty before any group exists; after a session is created it reports the setting name, session count, and started=false / port=-1 for an unlaunched process. (13/13)
• InterpreterRestApiTest#testGetInterpreterProcessStatus - endpoint returns a JSON array. (12/12)

Manually: run a paragraph, then curl -u <admin> http://localhost:8080/api/interpreter/status and confirm the interpreter appears with started: true. Restarting it from the setting page resets uptimeSeconds.

Screenshots (if appropriate)

N/A - REST only.

Questions:

• Does the license files need to update? No.
• Is there breaking changes for older versions? No - additive and read-only.
• Does this needs documentation? Can add a REST API doc section here if preferred, otherwise once the sibling sub-tasks land.