✨ [RUM-17561] add execution context tracking for main and renderer processes

[bcaudan]

Motivation

Electron apps run across a main process and one or more renderer processes, but RUM events carry no signal about which OS process instance produced them. That makes it hard to correlate events to a specific process run — especially across crashes/relaunches, or when a single long-lived process spans multiple RUM sessions. This adds execution_context tracking so every RUM event can be attributed to the process instance and session that produced it.

Changes

• Adds a new execution_context RUM event, scoped to one (process, session) pair: the same OS process across multiple sessions produces multiple execution_context documents, all sharing one stable instance_id (the OS pid) so they can still be correlated to the same process run.
• Tracks the main process via a session-scoped (fake view, main execution context) pair that's created together on init and on every session renewal, and closed together on session expiry. Renderer windows are tracked independently, one execution context per webContents, rotating on the same session boundaries and ending on real process destruction.
• Gated behind a new enableExecutionContext configuration flag. When enabled, the main process's real "main process" view is replaced by a lightweight fake view used to carry execution-context tagging, until main-process RUM events no longer need a view at all.
• Adds unit coverage for both halves' lifecycle and session-boundary rotation, plus e2e coverage for the full lifecycle (init, renderer open/close, session expiry/renewal, cross-tagging) and crash-file replay resolving to the execution context active when the crash actually happened.
• Adds a secondary renderer window to the playground app for manually exercising renderer-process execution contexts.

Test instructions

1. In playground/, run yarn dev (the playground's SDK init already sets enableExecutionContext: true).
2. Click "Stop Session", then click anywhere in the window to generate activity — this renews the session and rotates the main execution context.
3. Click the button to open the secondary renderer window; confirm it gets its own execution context, and that closing it ends that context cleanly.
4. Optionally, click "Crash" and let the app relaunch — the resulting error event should resolve to the execution context active when the crash happened, not the relaunched process's new one.

Checklist

• Tested locally (playground)
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated related documentation.
• Agentic code review findings addressed or explicitly dismissed.