Fix G1 RSL-RL config to use the registered environment name

[sylvesterkaczmarek]

Summary

rsl_rl_config() applies a tuned max_iterations=1000 schedule to several locomotion tasks, but the G1 entry is currently G1Joystick, which is not a registered MuJoCo Playground environment. The registered flat-terrain task is G1JoystickFlatTerrain, so that task misses the intended RSL-RL schedule and keeps the default max_iterations=100000.

This change:

• replaces the stale G1Joystick config key with the registered G1JoystickFlatTerrain name;
• leaves the existing RSL-RL hyperparameters unchanged;
• adds a regression test that verifies the environment is registered and receives max_iterations=1000.

Validation

• Audited the final branch diff: 2 files, +8/-1 lines, with no unrelated changes.
• Squashed to one focused commit: 76f5e8887445b4921b1ba91af50722ba6bcb4e66.
• Branch is based on current upstream main at e74217bb89c77a74ba02e4789263991864375799.
• Full tests were not run locally in this environment. The added regression test is included for upstream CI validation.