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.

tests/tools/test_computer_use.py

@@ -226,6 +226,21 @@ class TestDispatch:
         parsed = json.loads(out)
         assert "error" in parsed
 
+    def test_set_value_routes_to_backend(self, noop_backend):
+        """set_value must reach the backend — regression for missing _NoopBackend stub."""
+        from tools.computer_use.tool import handle_computer_use
+        out = handle_computer_use({"action": "set_value", "value": "Option A", "element": 5})
+        parsed = json.loads(out)
+        assert parsed.get("ok") is True
+        assert parsed.get("action") == "set_value"
+        assert any(c[0] == "set_value" for c in noop_backend.calls)
+
+    def test_set_value_missing_value_returns_error(self, noop_backend):
+        from tools.computer_use.tool import handle_computer_use
+        out = handle_computer_use({"action": "set_value"})
+        parsed = json.loads(out)
+        assert "error" in parsed
+
 
 # ---------------------------------------------------------------------------
 # Safety guards (type / key block lists)