prefixed collection index collides with root index in path helpers

[onmax]

Problem

When one collection uses the root index.md and another uses a prefixed index.md, Studio collapses both back to the same filesystem path.

Reproduction

Minimal repro:
onmax/repros/tree/main/nuxt-studio-index-collision

pnpm i && pnpm diagnose

Expected

• marketing/index.md resolves to index.md
• guides/guides/index.md resolves to guides/index.md
• getCollectionByFilePath('index.md') resolves marketing

Actual

marketing/index.md -> marketing/index.md -> index.md
guides/guides/index.md -> guides/guides/index.md -> index.md
getCollectionByFilePath('index.md') -> guides

This makes the prefixed collection win when Studio tries to recover a collection from index.md.