feat: simulate Flow triggers and actions for clipboard testing

[mcollovati]

Add a browserless simulation of Flow's client-side trigger/action API,
which is otherwise unreachable server-side. TriggerSimulation installs
a Triggers arming listener per mocked environment and fires armed
triggers, dispatching matched ActionSimulators. First slice covers the
clipboard: ClipboardSimulator exposes clipboard-write state, and
click() on Clickable/ContextMenuTester now fires client-side
triggers so context-menu actions run.

Installed and torn down in BaseBrowserlessTest,
BrowserlessApplicationContext and AbstractBrowserlessExtension.

[mcollovati]

Part of a 4-PR split (2 PRs per repo).

• Depends on feat: add test-support seam for simulating triggers and actions flow#25045 (the test-support seam). CI here will stay red until that flow PR is merged and a new 25.3-SNAPSHOT flow-server is published; verified locally against a local mvn -pl flow-server install of the seam.
• This is a clean regrouping of a larger branch. An earlier "install from MockVaadin.createUI" approach was reverted during development, so this PR has no net change to MockVaadin.kt.
• Verified locally: shared ClipboardSimulationTest 6/6, TriggerSimulationSerializationTest 1/1; spring & quarkus ClipboardSmokeTest 2/2 each.