Fix canonical native interactive button messages

[purpshell]

Native button sends currently use the old ButtonsMessage representation, which does not match the native InteractiveMessage shape that renders in the tested WhatsApp Web client. Emit version-1 native InteractiveMessage buttons so send transport selects the native_flow business metadata.

Support up to ten quick replies or three CTA buttons and reject reply/CTA mixtures. Align native body/title/footer limits with the typed sending contract. Native lists now match the existing API contract: thirty rows and its UTF-8 byte bounds, with optional section headings. Official list validation remains separate. Keep the existing builder signature. Conditional widget/Flow/mobile and Meta-server support are not claimed by this change.

Validation: go test ./... passed. Tests cover canonical shape, count/combination bounds and native transport/business-node selection. Actual pinned-backend encrypted Web acceptance is pending in the coordinated downstream repair cycle; source-only renderer fixtures are separate evidence.

Session provenance: reviewed partial checkpoint, commit 49c33837ba49bcb5442637cf203f8d99fadda6b9. Raw tool data and private contents are omitted explicitly. This record does not establish runtime acceptance or deployed availability.

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