Expose steam_boiler_formula on the Python bindings

The upstream cg crate gained `ComponentGraph::steam_boiler_formula`
in 0.5 (alongside the new `SteamBoiler` component category), but the
Python bindings never wired it up. Adds the binding and the matching
stub.

Signed-off-by: Sahas Subramanian <sahas.subramanian@proton.me>

Author: shsms

python/frequenz/microgrid_component_graph/__init__.pyi

@@ -360,6 +360,22 @@ class ComponentGraph(Generic[ComponentT, ConnectionT, ComponentIdT]):
                 are not wind turbines.
         """
 
+    def steam_boiler_formula(self, steam_boiler_ids: Set[ComponentIdT] | None) -> str:
+        """Generate the steam boiler formula for this component graph.
+
+        Args:
+            steam_boiler_ids: The set of steam boiler component IDs to include in
+                the formula.  If `None`, all steam boilers in the graph will be
+                included.
+
+        Returns:
+            The steam boiler formula as a string.
+
+        Raises:
+            FormulaGenerationError: if the given component IDs don't exist or
+                are not steam boilers.
+        """
+
     def grid_coalesce_formula(self) -> str:
         """Generate the grid coalesce formula for this component graph.
 

src/graph.rs

@@ -352,6 +352,18 @@ impl ComponentGraph {
             .map_err(|e| PyErr::new::<FormulaGenerationError, _>(e.to_string()))
     }
 
+    #[pyo3(signature = (steam_boiler_ids=None))]
+    fn steam_boiler_formula(
+        &self,
+        py: Python<'_>,
+        steam_boiler_ids: Option<Bound<'_, PyAny>>,
+    ) -> PyResult<String> {
+        self.graph
+            .steam_boiler_formula(extract_ids(py, steam_boiler_ids)?)
+            .map(|f| f.to_string())
+            .map_err(|e| PyErr::new::<FormulaGenerationError, _>(e.to_string()))
+    }
+
     fn grid_coalesce_formula(&self) -> PyResult<String> {
         self.graph
             .grid_coalesce_formula()