refactor: use formatNames utility for interface name generation (#14481)

## Changes

This PR consolidates the inline interface name generation logic in
`configToJSONSchema.ts` to use the existing `formatNames()` utility
function.

## Motivation

The inline code `singular(toWords(slug, true))` was duplicating logic
already available through `formatNames(slug).singular`. Using the
existing utility function:
- Reduces code duplication
- Maintains consistency with the rest of the codebase
- Makes it clear where the interface naming logic lives

## Testing

Verified that `formatNames(slug).singular` produces identical results to
the previous inline implementation across various slug patterns (plural,
singular, edge cases like 'media', 'series', etc.).

**No behavioral changes** - this generates the exact same interface
names for all collection and global slugs.