feat: improve dataservice OpenAPI response viewer

[Samuelfaure]

Summary

Improves the "Données renvoyées" OpenAPI response viewer on the dataservice detail page (API-6768):

• Each field's example now renders below its description — reading order is title → description → example → children.
• Darker description text (gray-800) and tree-node borders (gray-300) for better contrast.
• Per-node collapse/expand via a chevron, plus a global Ouvrir tout / Fermer tout control; default state is fully expanded.

Global open/close-all is coordinated through a provide/inject reactive signal (openApiCollapse.ts), so it broadcasts to every node across all endpoints without a path registry, while individual node toggles stay independent.

Test plan

Verified manually in the dev server against api-sirene-open-data (spec with nested objects and arrays):

• Reading order title → description → example on every node
• Description renders darker; borders are higher-contrast
• Per-node toggle is independent; leaf nodes have no chevron and stay aligned
• "Fermer tout" collapses every node across all endpoints; "Ouvrir tout" expands them all; default on load is fully expanded
• Flat/empty schema hides the global control (code-verified via hasExpandableNode; not exercised against a real flat schema)

nuxt typecheck and eslint both pass.

Linear: https://linear.app/pole-api/issue/API-6768/ameliorer-lui-de-visualisation-des-donnees