fix(catalog): document DataForSEO Google AI Mode live as single-task

[stonexer]

What this does

Stops catalog_get from telling agents they can POST multiple tasks to dataforseo.x.serp-google-ai-mode-live-advanced (/serp/google/ai_mode/live/advanced).

Same class as dataforseo.web.backlinks.summary (treg feedback #102 / #487): this is a Live single-task route. Extra array elements come back as per-task status 40000 "You can set only one task at a time" behind HTTP 200 (first task OK, the rest rejected at $0). The endpoint's input.note still said "the POST body is an ARRAY of task objects with these fields — one object per task", which agents read as multi-task batching. Provider-level limits already name Live = one task per POST; this endpoint's own note did not.

Catalog-only: rewrite input.note to name the single-task cap and the 40000, including that HTTP stays 200. No settlement change, no auto-split of multi-task arrays into billed calls.

Addresses treg feedback #94.

How it was tested

• YAML note on dataforseo.x.serp-google-ai-mode-live-advanced now mirrors backlinks.summary / google.serp.organic / page.audit: exactly one task object; extra elements get 40000.
• test_google_ai_mode_live_documents_single_task_constraint locks the YAML note, 40000, and one-element test_request.
• test_catalog_get_dataforseo_ai_mode_live_names_the_single_task_limit locks the catalog_get HTTP payload.
• uv run --frozen python scripts/catalog_validate.py — 102 provider files, 0 errors.
• uv run --locked lint-imports — 14 contracts kept.
• uv run --with pytest-xdist pytest -n auto -q — 4171 passed, 8 skipped.

Checklist

• uv run --with pytest-xdist pytest -n auto -q passes locally
• Added or updated tests for the change (if it affects behavior)
• Updated the relevant docs/context/ fragment (docs/context/architecture/catalog.md)
• No secrets in the diff (keys, tokens, .env values)