Is signaling to those behind me to complete the row, or to those beside me to close a gap, considered idle talk during Friday prayer, in accordance with the hadith, "Whoever touches pebbles has engaged in idle talk," and what should I do in such a situation?

There is no harm in light gesturing during prayer out of need. However, speaking during prayer invalidates it. As for before entering prayer, there is no harm in speaking or gesturing before the sermon or after its conclusion, up until the intention for prayer (ihram (definition)). But during the sermon, speaking is not permissible, though gesturing to prohibit speech is allowed.