Fix dropdown field change API to sync selection with model updates

[vivekjyani]

Context / Spec

When a dropdown field value is updated externally via the change API (for example, programmatic model updates or mirrored-field updates), DropdownView could show stale selection state because its internal @State (selectedDropdownValueID) only captured the initial model value.

Ticket: NO-2072

What Changed

1) State sync fix in DropdownView
File: Sources/JoyfillUI/View/Fields/DropdownView.swift

Added an observer to sync internal selection state whenever dropdownDataModel.dropdownValue changes externally:

.onChange(of: dropdownDataModel.dropdownValue) { newValue in
    if selectedDropdownValueID != newValue {
        selectedDropdownValueID = newValue
    }
}

The guard prevents redundant writes and avoids re-triggering the existing onChange flow unnecessarily.

2) UI regression test for mirrored dropdown behavior
File: JoyfillSwiftUIExample/JoyfillUITests/ChangeUITests/OnChangeHandlerUITests.swift

Added testDropdownCoordinatess() (iPad-only) that:

• navigates to the relevant page setup,
• selects "No" in the left dropdown,
• asserts the right dropdown reflects the same value.

This validates that external/model-driven dropdown updates are now reflected in UI state.

Design Decisions

• Why .onChange on model value? It is the minimal fix that preserves current architecture while correctly re-syncing local @State on external updates.
• Why guard equality before assignment? Prevents unnecessary state writes and avoids duplicate event propagation.
• Why iPad-only test guard? The test scenario relies on the iPad-specific layout/navigation path used by existing change-handler UI tests.

Public API Impact

No public API changes. This is an internal state-sync and test coverage update.

Test Coverage

Added UI coverage via:

• testDropdownCoordinatess() in OnChangeHandlerUITests

Reviewer Notes

• Scope is focused: 2 files changed (runtime fix + regression test).
• Fix addresses a SwiftUI state-sync edge case where @State initialized in init does not automatically track subsequent model updates.