fix(combobox): prevent collapse to 1px in flex align-items:center containers

[yordan-st]

Summary

• Fixes WC-3409: Combobox collapses to 1px when placed in a flex row container with align-items: center
• Root cause: Mendix injects .form-group.no-columns (column flex, no explicit width) between the user's container and .widget-combobox. flex-grow: 1 distributes height in a column container, not width. Children suppress min-width to ~0 via min-width: 0 on .widget-combobox-selected-items (intentional, needed for multiselect tag wrapping)
• Fix: min-width: 15ch on .widget-combobox — explicit value does not ask children, breaking the collapse chain. ch scales with font-size rather than using px

Changes

• src/ui/Combobox.scss — min-width: 15ch on .widget-combobox root rule
• src/__tests__/ComboboxStyles.spec.ts — unit test updated to assert 15ch
• e2e/Combobox.spec.js — E2E regression tests added (pending Studio Pro test page at /p/combobox-flex-layout)
• openspec/changes/fix-combobox-flex-collapse/ — design artifacts with corrected root cause analysis

Test plan

1. Create a new page in Studio Pro with a container whose CSS class applies display: flex; flex-flow: row; align-items: center (e.g. add a class row-center and define it in a custom stylesheet).
2. Place a Combobox widget (single-select, connected to any enumeration attribute) inside that container.
3. Run the app locally and open the page.
4. Before the fix: the combobox is invisible (collapses to ~1px width). After the fix: the combobox renders with a visible width (≥ 15ch).
5. Repeat steps 1–4 with a multi-select Combobox to confirm the same result.
6. Verify that typing in the combobox input and selecting a value still works normally.

Override

To restore previous behavior: .widget-combobox { min-width: 0; } in app CSS.