Fix fullscreen Steam game window rules

[flrsn]

Summary

• apply fullscreen idle inhibition to the existing steam.* rule, covering Steam, class-steam games, and steam_app_* games
• add fullscreen idle inhibition for games launched through gamescope
• scope floating behavior to the titled Steam main and Friends windows so games exposing class steam are not forced to float
• add a regression test for the reproduced Steam and gamescope classes

Fixes #6947

Scope

This covers the concrete steam, steam_app_*, and gamescope classes reported in the issue. Native Linux games may expose arbitrary app-specific classes, which cannot be identified reliably by a static Steam class rule.

Testing

• test/shell.d/steam-test.sh
• bash test/shell.d/hyprland-default-config-test.sh
• git diff --check

The full test suite was also exercised while developing the PR. Its 10 unrelated environment-dependent failures reproduced unchanged on an untouched origin/quattro worktree.