Fix Falabella order filters, Substack cursor paging and empty Gemini answers - #1628

Summary

Three adapter bugs seen in Cloud tool errors on 11-10, all in the adapter JSON we ship.

• Falabella Seller Center get_orders: the sort_by enum offered createdat / updatedat and the status enum readytoship. Falabella refuses them (E075: Invalid sort filter on 6 of 6 calls, E036: Invalid status filter on 3 of 3). The GetOrders reference documents created_at / updated_at and pending, canceled, ready_to_ship, delivered, returned, shipped, failed.
• Substack list and search: Substack changed its archive endpoint. It now answers { posts, nextCursor } instead of an array, refuses any offset (even 0) and refuses limit above 25. Our excludes ([*].body_html …) no longer matched, so full post bodies went back to the model, and every second-page call failed. Checked live on three publications today.
• Gemini generate_text / generate_content / analyze_file_url: a candidate without content.parts (safety stop, empty answer) made join() fail on null, and the raw response went back unshaped (about once a day).

Changes

• falabella-seller-center.json: correct sort_by and status enums (adds failed).
• substack.json: limit maximum 25, offset replaced by cursor (the previous nextCursor), excludes now under posts[*], descriptions and instructions explain the new paging.
• gemini.json: join('', … .text || \[]`)` in the three mappings.
• Specs: enum check for Falabella; Substack shape, cursor and limit checks (static) and the live smoke test moved to the new shape; Gemini mapping without parts.

Description and instruction changes sync on their own. The parameter changes (Substack cursor, the enums) are structural, so installed connectors get them after "Apply update"; new installs get them at once.

Type

• Bug fix
• New feature
• Documentation
• Refactoring
• Performance improvement
• Other (describe below)

Testing

• Added/updated unit tests
• Tested manually (describe steps below)
• No tests needed (explain why)

In packages/backend (Prisma client generated in the worktree):

• npx jest src/adapters/intl/substack src/adapters/intl/gemini src/adapters/intl/falabella-seller-center: 4 suites, 71 passed (8 live tests skipped).
• RUN_SUBSTACK_LIVE=1 npx jest src/adapters/intl/substack: 25 passed against lennysnewsletter.com.
• npx eslint on the three specs: clean.
  From the root: node scripts/validate-adapters.mjs --warn (no warning for these adapters), node scripts/regenerate-catalog.mjs (no diff), node scripts/adapter-count.mjs --check OK.

Checklist

• My code follows the existing code style
• I have tested my changes locally
• I have updated documentation (if applicable)
• All existing tests pass (npm test) (adapter suites locally, full suite in CI)
• This PR has a descriptive title

Same behaviour in Cloud and self-hosted.

Related Issues

None (found in the 11-11 daily check).