fix(computer-use): add set_value to ComputerUseBackend ABC and _NoopBackend stub

_dispatch() routes action="set_value" to backend.set_value(), but:
- ComputerUseBackend did not declare set_value as @abstractmethod, so
  subclasses could silently omit it without a TypeError at class load time.
- _NoopBackend (the test/CI stub) had no set_value method at all, causing
  AttributeError in any test that exercises the set_value action path.

Fix:
- Add set_value as @abstractmethod to ComputerUseBackend in backend.py.
- Add a recording stub in _NoopBackend in tool.py.
- Add two TestDispatch cases: one verifying the call reaches the backend,
  one verifying the missing-value guard returns a clean error.

tools/computer_use/tool.py

@@ -200,6 +200,10 @@ class _NoopBackend(ComputerUseBackend):  # pragma: no cover
         self.calls.append(("focus_app", {"app": app, "raise": raise_window}))
         return ActionResult(ok=True, action="focus_app")
 
+    def set_value(self, value: str, element: Optional[int] = None) -> ActionResult:
+        self.calls.append(("set_value", {"value": value, "element": element}))
+        return ActionResult(ok=True, action="set_value")
+
 
 # ---------------------------------------------------------------------------
 # Dispatch