BUG: add to_dict/from_dict to RingClusterMotor for serialization round-trip

RingClusterMotor defined neither to_dict nor from_dict, and its base class
Motor (ABC) has none either (only GenericMotor does). On decode, RocketPyDecoder
fell through to instantiating the class from the filtered dict, but the inherited
Motor.to_dict serializes computed motor fields (thrust_source, inertias, ...) and
none of the constructor arguments (motor, number, radius). Reconstruction called
RingClusterMotor() with no args:

    TypeError: RingClusterMotor.__init__() missing 3 required positional
    arguments: 'motor', 'number', and 'radius'

This TypeError is not caught by the decoder (only ImportError/AttributeError are),
so any Rocket/Flight carrying a RingClusterMotor could not be reloaded, which also
breaks Monte Carlo caching and .rpy save/load for the new motor.

The inner motor is a RocketPy object and is serialized/reconstructed recursively,
so from_dict just forwards the reconstructed motor plus number and radius.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Gui-FernandesBR

rocketpy/motors/ring_cluster_motor.py

@@ -291,6 +291,25 @@ def info(self, *args, **kwargs):
         print(f" - Radial Distance: {self.radius} m")
         return self.motor.info(*args, **kwargs)
 
+    def to_dict(self, **kwargs):
+        data = super().to_dict(**kwargs)
+        data.update(
+            {
+                "motor": self.motor,
+                "number": self.number,
+                "radius": self.radius,
+            }
+        )
+        return data
+
+    @classmethod
+    def from_dict(cls, data):
+        return cls(
+            motor=data["motor"],
+            number=data["number"],
+            radius=data["radius"],
+        )
+
     def draw_cluster_layout(self, rocket_radius=None, show=True):
         """Draw the geometric layout of the clustered motors."""
         fig, ax = plt.subplots(figsize=(6, 6))