[FEATURE] Add filter_link_idx to ContactForceSensor.

[jsw7460]

ContactForceSensor now accepts filter_link_idx, mirroring ContactSensor: contacts with the sensor link whose other participant is one of the listed links are excluded from the reported force. The filter is applied on both ground-truth-update paths; the vectorized zerocopy path and the _kernel_get_contacts_forces kernel.
Adds test_contact_force_sensor_filter_link_idx.

Description

ContactForceSensor now accepts filter_link_idx (default ()), mirroring
ContactSensor from #2655: contacts with the sensor link whose other
participant is one of the listed links are excluded from the reported force.

- `_kernel_get_contacts_forces` (the kernel fallback): takes
  `filter_links_idx` and skips a contact for a sensor when the counterpart
  is in that sensor's filter list;
- `filtered_sensor_idx` gates the comparison so sensors without a filter
  keep the cheap path (same optimisation as `ContactSensor`).

Related Issue

Resolves #2771

Motivation and Context

ContactSensor already filters its boolean by counterpart link, but ContactForceSensor only reported the total contact force on the link, with no way to scope it (e.g. "force on a foot from the ground, excluding self-contact").

How Has This Been / Can This Be Tested?

  pytest tests/test_sensors.py::test_contact_force_sensor_filter_link_idx tests/test_sensors.py::test_contact_sensor_filter_link_idx

test_contact_force_sensor_filter_link_idx puts a box on the floor (env 0) / a box on the floor with another box stacked on top (env 1), with two ContactForce sensors on the bottom box — one plain, one with filter_link_idx (floor.link_start,). It asserts the filtered sensor reads ~0 in env 0 and reads only the top box's downward push in env 1, while the unfiltered sensor reads the net contact force in both.

Checklist:

• I read the CONTRIBUTING document.
• I followed the Submitting Code Changes section of CONTRIBUTING document.
• I tagged the title correctly (including BUG FIX/FEATURE/MISC/BREAKING)
• I updated the documentation accordingly or no change is needed.
• [ x] I tested my changes and added instructions on how to test it for reviewers.

• I have added tests to cover my changes.
• All new and existing tests passed.

[Milotrince]

This feature has already been implemented by #2655 .

[jsw7460]

@Milotrince
Hi, thank you for #2655 !
But #2655 added filter_link_idx only to gs.sensors.Contact (the boolean contact sensor), not to gs.sensors.ContactForce (the force-vector sensor).

What I'm after here is a ContactForce sensor that sums only the force from contacts with a given subset of links — e.g. "force on a foot from the ground, excluding self-contact" — which
isn't possible today since ContactForce reports the total contact force on the link.