fix(graphql): blocks return null with select: true (#15464)

### What

GraphQL queries with `select: true` were returning null for block fields
and throwing "Cannot return null for non-nullable field" errors.

### Why

The selection builder wasn't nesting block field selections under their
block slug when processing inline fragments. It was creating `{
contentField: { text: true } }` instead of `{ contentField: { content: {
text: true } } }`, so the database projection couldn't find the fields.

### How

- Added `blockSlug` to GraphQL block type extensions so we can identify
block types
- Updated inline fragment handling to nest selections under the block
slug for union types

Fixes #15460