feat(opencode): bridge question tool to ACP elicitation - #42104

Summary

• Adds packages/opencode/src/acp/elicitation.ts — a handler (modelled on permission.ts) that translates the question tool part into an unstable_createElicitation ACP request and maps the client response back to sdk.question.reply/reject
• Wires the handler into ACPEvent.Subscription.handleToolPart — when toolName === "question" and the state is running, the elicitation handler fires instead of the normal runningTool path
• Extends the Connection type in both event.ts and service.ts to include unstable_createElicitation
• Preserves the auto-reject fallback when the client does not advertise elicitation support

Test plan

• Accepted elicitation — form schema is correct, answers are mapped back and sdk.question.reply is called
• Declined elicitation — sdk.question.reject is called
• Cancelled elicitation — sdk.question.reject is called
• Client without elicitation support — auto-rejects immediately without calling unstable_createElicitation
• unstable_createElicitation throws — falls back to reject
• Multiple questions in one tool call — all map into a single form with one property per question
• Multi-select question (multiple: true) — uses array schema with anyOf items
• Unknown session — handler exits early, no calls made
• Full ACP test suite passes (136 tests)

Fixes #38121