Fixes #20697: Ensure unique signal handlers for counter models

[pheus]

Fixes: #20697

Summary
Prevent double counting in CounterCacheField by ensuring signal receivers are connected once per sender.

Implementation

• Update utilities.counters.connect_counters() to use per‑sender dispatch_uid for post_save, pre_delete, and post_delete.
• Guard against reconnecting by tracking models already processed during registration.
• Leave a single receiver to update all registered counters for the same sender (e.g., device_count and member_count).

Impact

• Fixes +2/−2 updates when multiple counters share the same child model (e.g., Device).
• No database changes; backward‑compatible internal wiring change.

[pheus]

Note: This bug only manifests when multiple counters are registered for the same model/sender. That isn’t the case in core yet, so I didn’t add a failing test here without introducing test‑only fields. Once the PR for #19523 is ready, it will include a second counter for Device, which gives us a clean repro to test against. I’m happy to add/enable the test in that PR (or as a follow‑up). I hope that’s acceptable.

[pheus]

I can push my current WIP for device_count and module_count as a draft PR to make it easier to review this change. Let me know if that would be helpful.

[jeremystretch]

Thanks @pheus!