feat(web): show the User Input default value beneath the field name with a working tooltip

[dparkmit24]

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Summary

Fixes #39563.

In the User Input (Start) node field list, a field's default value is now shown beneath the field name when one is set, so you can see at a glance which fields will fall back to a default at run time instead of opening each field's edit panel one at a time. This is a display-only change — defaults already apply correctly at run time; this just surfaces an existing property in the list.

Two problems with a first pass at this (found by browser-testing, not visible in unit tests) motivated the final shape:

1. Layout — the indicator first rendered inline in the row's right-hand metadata zone next to required. It now sits on its own line beneath the {icon} name · label identity line, left-aligned, in the row's persistent left column. required stays top-right. Single-line rows with no default keep their original height and don't shift on hover.

2. Tooltip — the value is truncated when long, with the full value on hover. The first pass used the native title attribute, which disappeared on hover: the metadata zone only renders while the row is not hovered, so hovering to read the tooltip flipped the row's hover state and unmounted the indicator. Moving the indicator into the persistent column fixes the unmount, and the tooltip now uses the house @langgenius/dify-ui/tooltip (portal-based), which survives the row re-render.

The nullish/empty guard is unchanged: false and 0 are shown; an unset or empty-string default shows nothing.

Tests assert position (beneath the name, not in the required zone) and tooltip content (full value on hover, trigger stays mounted) — not just presence, since presence-only tests missed both problems above.

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods