refac: use backtick name style in PiecewiseFormulation repr

Match Constraint repr pattern: `name` instead of 'name'.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: FBumann

linopy/piecewise.py

@@ -77,7 +77,7 @@ class PiecewiseFormulation:
     constraints: Constraints
 
     def __repr__(self) -> str:
-        r = f"PiecewiseFormulation '{self.name}' — {self.method}\n"
+        r = f"PiecewiseFormulation `{self.name}` — {self.method}\n"
         r += "  Variables:\n"
         for vname, var in self.variables.items():
             dims = ", ".join(str(d) for d in var.coords) if var.coords else ""