fix: correct type assertion in stateMapper test

The test for unknown result subtype was using an invalid subtype value
directly, which caused a TypeScript error. Changed to use a type
assertion to properly test the unknown subtype case, matching the
pattern used for testing unknown message types.

This fix ensures all TypeScript checks pass whilst still testing the
edge case behaviour for unexpected subtype values.

src/lib/utils/stateMapper.test.ts

@@ -267,10 +267,10 @@ describe("stateMapper", () => {
     });
 
     it("returns null for result with unknown subtype", () => {
-      const message: ClaudeStreamMessage = {
+      const message = {
         type: "result",
         subtype: "unknown_type",
-      };
+      } as unknown as ClaudeStreamMessage;
       expect(mapMessageToState(message)).toBeNull();
     });