test(opencode): cover native reasoning continuation

[kitlangton]

Summary

• add a small native-session test DSL for stored history and OpenAI Responses body expectations
• cover store:false continuation behavior for bare reasoning ids without encrypted state
• cover encrypted OpenAI reasoning state preservation through native request lowering

Verification

• bun test --timeout 5000 test/session/llm-native.test.ts
• bun typecheck (packages/opencode)
• bunx prettier --check test/session/llm-native.test.ts
• git diff --check
• bun turbo typecheck (pre-push hook)

Note: scoped oxlint reports existing warnings in this file, 0 errors.