fix: derive Default for ProxyType in testnet runtime

Fixes clippy::derivable_impls warning by deriving Default instead of
manually implementing it.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: drewstone

runtime/testnet/src/lib.rs

@@ -1142,6 +1142,7 @@ parameter_types! {
 
 /// The type used to represent the kinds of proxying allowed.
 #[derive(
+	Default,
 	Copy,
 	Clone,
 	Default,