Fix flex contact filtering compaction

[fallintoplace]

Fixes #3297.

This changes the flex contact filtering pass so it no longer reorders contacts[] while the farthest-point sampling loop is still using index-based selection state.

Before this change, mj_filterFlexContacts could keep a different set of contacts than the one the selector actually picked. In the bad cases, the retained prefix after filtering could drop or replace the final selected contact.

What changed:

• keep the contacts selected by FPS in a temporary buffer
• compact the filtered prefix once at the end of the loop
• add a regression test that compares the helper against a small reference FPS implementation

Validation:

• ran ../build/bin/engine_collision_driver_test from test/