fix: prevent horizontal scroll jump when opening a promotion parameter dropdown

[mvanhorn]

📝 Description

Opening a promotion parameter dropdown in the workflow view scrolls the page horizontally back to the pipeline's left edge, because the dropdown's focus() call triggers the browser's default scroll-into-view on the whole diagram. This change restores focus without the jump: target_params.js now focuses the select and text inputs with preventScroll: true and preserves the caret position via setSelectionRange instead of relying on the default focus scroll.

Added mocha specs in switch.spec.js that stub HTMLElement.prototype.focus and assert both the select and text parameter inputs are focused with preventScroll: true, and that the caret restore happens through setSelectionRange.

Fixes #1003

✅ Checklist

• I have tested this change
• This change requires documentation update