Document setting ticket priority on the update endpoint (Preview)

[Apexinsideapex]

Why?

Priority can already be set on a conversation through the API, but not on a ticket, so an integration that triages tickets in its own tooling had nothing to write back to. This documents the other half.

How?

Adds the priority level to the Preview update-ticket request body and example, and declares the two error responses the field can produce.

Notes for reviewers

• Affected versions: Preview only. No dated version changes.
• The spec addition is byte-identical to the developer-docs copy — diffed to confirm the two stay in step.
• Tickets already document a top-level attribution field, so unlike the conversation change this adds one property, not two.
• 403 and 422 were not declared on this operation before. The codes and messages match what the endpoint actually returns.
• The ticket response has no priority field, so this covers the write only.
• Rebased on main after the two schema additions that landed today.

_{Generated with Claude Code}