test(opencode): cover message ID rollover loop - #150

Summary

• add an exact regression for the August 14 message-ID rollover
• construct an older completed turn whose IDs sort above a newer user message
• prove the prompt loop reaches model lookup for the newer turn instead of silently exiting

This is the test-only follow-up to #149 and addresses the P3 Cubic review note. It does not change production behavior.

Verification

• bun test test/session/prompt.test.ts -t 'nonmonotonic message IDs|message ID rollover' — 2 passed
• bun typecheck in packages/opencode — passed
• repository filtered typecheck in the push hook — 17/17 passed

Summary by cubic

Adds a regression test for the August 14 message‑ID rollover to ensure the prompt loop starts a new user turn instead of silently exiting. Previously the loop could exit when older completed turn IDs sorted above a newer user message; the test proves it now reaches provider model lookup for the newer turn.

Reviewer notes

• Adds rolloverNoLLMServer test case in packages/opencode/test/session/prompt.test.ts.
• Mocks a blocking provider via ProviderSvc.getModel to signal when model lookup begins.
• Constructs an older completed assistant turn whose IDs sort above a newer user message and asserts prompt.loop starts the newer turn.
• Test-only change; no production code or behavior modified.

^{Written for commit dbb9ba9. Summary will update on new commits.}