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

[PatrikKozak]

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

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🚀 This is included in version v3.75.0