fix(connection): snapshot Date so stale heartbeat check protects against useFakeTimers

[vkarpov15]

Fix #14949

Summary

In our jest docs https://mongoosejs.com/docs/jest.html we recommend importing mongoose before calling useFakeTimers(), which gives us a way to work around #14949: ensure we get the Node built in Date class before useFakeTimers clobbers it. Works well with our claim that importing mongoose before useFakeTimers is "Fine for basic cases"

Examples

[Copilot]

Pull Request Overview

Snapshots the native Date constructor in connection.js to avoid Jest useFakeTimers() and similar timer mocks interfering with the stale heartbeat check.

• Snapshot Date from the global object at module load.
• Add a comment explaining the intent.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}