[FEATURE] Thread Diagnostics APIs

Problem

Thread diagnostics (full JVM thread dump with stack traces, lock info, deadlock detection, and system startup info) are currently only accessible via DWR calls to ThreadMonitorTool. The DWR returns an HTML-formatted <pre> string which limits filtering and search capabilities. Need REST endpoints to answer "What's happening inside the JVM?" with structured JSON data.

Goal

Provide REST endpoints for full thread dumps as structured JSON (enabling frontend filtering, search, deadlock highlighting) and lightweight system/thread count information.

APIs

• GET /v1/maintenance/_threads — Full thread dump as structured JSON
• GET /v1/maintenance/_threads/info — System startup time and thread counts

Personas

• Developer Teams — Need REST APIs for Angular Threads tab
• System Administrators — Need to diagnose thread contention, deadlocks, and resource usage

Legacy Reference

• DWR Class: com.dotcms.cmsmaintenance.ajax.ThreadMonitorTool
• JSP: view_cms_maintenance.jsp lines 683–757 (getAllThreads, getSysInfo)
• Target Class: MaintenanceResource.java (/api/v1/maintenance)

Start with MaintenanceResource.java under /api/v1/maintenance, then compare the legacy ThreadMonitorTool DWR methods getAllThreads and getSysInfo in view_cms_maintenance.jsp lines 683–757. Implement the two listed REST endpoints with structured JSON for the full thread dump and system/thread counts; done means both APIs expose the diagnostics needed by the Angular Threads tab without the HTML-formatted DWR response.