Add /idle endpoint for hosting platform idle tracking

[isabelizimm]

Opening duplicate PR based off these instructions

BELOW COPIED FROM #13146

---

Summary

Adds a /idle HTTP endpoint on the Positron server/REH build that Posit Cloud can query to determine how long the user has been idle.

Architecture

The existing IUserActivityService keeps track of DOM events (e.g. keydown, mousedown, touchstart. A new browser-side workbench contribution, PositronIdleReporterContribution, registers a callback to watch activity changes, upon which it forwards timestamps to a new IPC channel every 30s. A new service, PositronIdleTrackingService, keeps the most recent timestamp monotonically and exposes the result via the HTTP endpoint.

Response format

GET /idle?tkn=<connection-token>

{
  "seconds_idle": 142,
  "last_activity_epoch_ms": 1718900000000
}

Behavior notes

Since timestamps are sent every 30 seconds, the idle time may be reported up to 30s even while Positron is in use.

Opening a tab counts as activity for ~60-70s. IUserActivityService initializes isActive = true on construction, treating the act of opening the window as an implicit activity signal (see the service's own comment). The bundled DomActivityTracker then verifies continued activity via DOM events: after 2 poll intervals (~60s) with no keydown/mousedown/touchstart, plus a 10s debounce, it flips to isActive = false.

In practice: if a user opens a tab and walks away without interacting, seconds_idle will report 0 for approximately 60-70 seconds before it begins to climb. This is expected and acceptable for session-management use cases where idle timeouts are on the order of minutes or hours, but it's worth noting for any consumer that expects sub-minute precision.

Release Notes

New Features

• N/A (internal server/REH feature; no user-visible change)

Bug Fixes

• N/A

QA Notes

@:sessions

This feature only activates in the server/REH build and requires Linux (license manager is Linux-only).

Automated tests

```bash
./scripts/test.sh --run src/vs/platform/positronIdleTracking/test/node/positronIdleTrackingService.test.ts
./scripts/test.sh --run src/vs/platform/positronIdleTracking/test/common/positronIdleTrackingIpc.test.ts
```

Manual verification (Linux only)

1. Build and start the server:
   ```bash
   npm run build-start && npm run build-check
   ./scripts/code-server.sh --connection-token test-token --license-key-file /path/to/license.lic
   ```
2. Open the URL printed by the server in a browser.
3. Wait ~10s, then query `/idle`:
   ```bash
   curl -s "http://localhost:/idle?tkn=test-token" | jq .
   ```
4. Verify:
   • `seconds_idle` near 0 when interacting
   • `seconds_idle` oscillates between 0 and ~30s when actively interacting (heartbeat interval)
   • `seconds_idle` grows unboundedly when the user stops interacting (after the ~60-70s grace period described above)
   • HTTP 403 when `?tkn=` is missing or wrong

🤖 Generated with Claude Code

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:sessions

^{readme  valid tags}

[isabelizimm]

Copying comments from other PR:

@jmcphers : This checks user activity but not kernel activity; i.e. if you start a long-running computation and close the browser tab, your session will quickly begin reporting as idle even though Python or R are still doing work. Is that intended behavior for this endpoint?

@mslynch : Yes - we're planning on calling the Kallichore API as well for that.