Fix machine envelope G-code emitting wrong limits due to broken extruder_id indexing (#12414)

print_machine_envelope() used get_extruder_id(extruder_id)*2 to index
machine limit arrays that only hold [Normal, Stealth] (2 entries).
For multi-extruder setups this went out-of-bounds, causing wrong M201/M203
values in the G-code which then override the estimator's correct limits.

Same class of bug as c6d1c11ebb but on the G-code writer side.

Changes:
- Remove unused extruder_id param from print_machine_envelope()
- Use .values.front() for M201/M203, matching M204/M205 in same function
- Change get_option_value() fallback from .back() to .front() so any
  future out-of-bounds index returns Normal mode instead of Stealth

src/libslic3r/GCode/GCodeProcessor.cpp

@@ -124,7 +124,7 @@ static void set_option_value(ConfigOptionFloats& option, size_t id, float value)
 static float get_option_value(const ConfigOptionFloats& option, size_t id)
 {
     return option.values.empty() ? 0.0f :
-        ((id < option.values.size()) ? static_cast<float>(option.values[id]) : static_cast<float>(option.values.back()));
+        ((id < option.values.size()) ? static_cast<float>(option.values[id]) : static_cast<float>(option.values.front()));
 }
 
 static float estimated_acceleration_distance(float initial_rate, float target_rate, float acceleration)