fix(llm): preserve native continuation metadata - #28678

Summary

• preserve provider-owned reasoning metadata through native opencode request lowering
• support OpenAI Responses encrypted reasoning continuation while avoiding non-persisted rs_* ids with store:false
• add Anthropic user image continuation lowering and shared media helpers
• move golden recorded scenarios onto a small generated-conversation DSL and add a focused OpenAI reasoning-continuation cassette

Verification

• bunx prettier --write packages/llm/src/protocols/anthropic-messages.ts packages/llm/src/protocols/openai-responses.ts packages/llm/src/protocols/shared.ts packages/llm/test/continuation-scenarios.ts packages/llm/test/provider/anthropic-messages.test.ts packages/llm/test/provider/golden.recorded.test.ts packages/llm/test/provider/openai-responses.test.ts packages/llm/test/recorded-golden.ts packages/llm/test/recorded-scenarios.ts packages/opencode/src/session/llm/native-request.ts packages/opencode/test/session/llm-native.test.ts
• git diff --check
• bunx oxlint packages/llm/src/protocols/anthropic-messages.ts packages/llm/src/protocols/openai-responses.ts packages/llm/src/protocols/shared.ts packages/llm/test/continuation-scenarios.ts packages/llm/test/provider/anthropic-messages.test.ts packages/llm/test/provider/golden.recorded.test.ts packages/llm/test/provider/openai-responses.test.ts packages/llm/test/recorded-golden.ts packages/llm/test/recorded-scenarios.ts packages/opencode/src/session/llm/native-request.ts packages/opencode/test/session/llm-native.test.ts (known pre-existing warnings in anthropic/opencode test files)
• bun typecheck (packages/llm)
• bun typecheck (packages/opencode)
• bun test --timeout 5000 test/provider/openai-responses.test.ts (packages/llm)
• bun test --timeout 5000 test/provider/anthropic-messages.test.ts (packages/llm)
• bun test --timeout 5000 test/provider/golden.recorded.test.ts (packages/llm)
• bun test --timeout 5000 test/session/llm-native.test.ts (packages/opencode)
• RECORD=true RECORDED_TEST=reasoning-continuation bun test --timeout 120000 test/provider/golden.recorded.test.ts (packages/llm)
• pre-push hook: bun turbo typecheck