spec-sync(v2): document extract options.grounding (unwired, use extra_body)

[yzld2002]

Automated V2 spec-sync PR (client.v2).

• Commit 1 (mechanical): normalized V2 spec snapshot + regenerated reference models.
• Commit 2 (AI, only if the spec diff needs SDK changes): client.v2 resources/methods/tests/docs wired from the diff, added after this PR opened. Workflow-only drift is excluded and an AI no-op is skipped, so some drifts produce a mechanical-only PR with no second commit.

Gates (surface-lock, V2 contract tests, lint/test/typecheck) must pass. When present, the AI commit is a draft a human finishes (the V2 ergonomic layer — unified Job, dual-host, schema coercion — is not in the spec). Human review required before merge.

What changed

AI-generated from the PR diff — verify against the actual changes.

This PR re-adds the options.grounding field to the extract spec but keeps it unexposed as SDK surface, reachable only via extra_body.

Changes:

• Spec now documents options.grounding (Preview boolean, default true) on the extract request schema across sync/async and JSON/form-data routes.
• extract continues to expose only strict as a top-level keyword shorthand; grounding remains unwired pending a joint decision with ade-typescript.
• Docs updated to show reaching grounding via extra_body={"options": {"grounding": ..., "strict": ...}}, noting the merge is shallow at the top level.
• New tests pin that the SDK's own options payload carries only strict, and that grounding is reachable solely through extra_body.

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