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/backend.py

@@ -142,6 +142,14 @@ class ComputerUseBackend(ABC):
     def focus_app(self, app: str, raise_window: bool = False) -> ActionResult:
         """Route input to `app` (by name or bundle ID). Default: focus without raise."""
 
+    # ── Native-value mutation ────────────────────────────────────────
+    @abstractmethod
+    def set_value(self, value: str, element: Optional[int] = None) -> ActionResult:
+        """Set a native value on an element (e.g. AXPopUpButton selection).
+
+        `element` is the 1-based SOM index returned by a prior capture call.
+        """
+
     # ── Timing ──────────────────────────────────────────────────────
     def wait(self, seconds: float) -> ActionResult:
         """Default implementation: time.sleep."""

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