[Blueprints] Allow WXR imports to skip attachment fetching

[adamziel]

What?

Adds a fetchAttachments option to the importWxr step.

fetchAttachments	Behavior
true or omitted	Keep the current behavior: fetch attachment files referenced by the WXR.
false	Ask the importer to skip fetching remote attachment files.

The public Blueprint schema is regenerated to include the new option.

Why?

Blueprint v2 models this as staticAssets: "fetch" | "hotlink". The future v2 runner can map staticAssets: "hotlink" to fetchAttachments: false, but the low-level WXR step needs the importer control first.

This stays separate from SQL URL rewriting and from broader WXR controls like author mapping, comments, users, site options, and explicit URL maps.

Testing

• npm exec nx run playground-blueprints:test:vite -- --testFiles=packages/playground/blueprints/src/tests/steps/import-wxr.spec.ts
• npm exec nx run playground-blueprints:typecheck
• npm exec nx run playground-blueprints:lint
• npm exec nx run playground-blueprints:build:blueprint-schema
• npm run format:uncommitted
• git diff --check

Part of #2592.