fix(storage): stream large Codex rollout imports

[liuxiaocs7]

Summary

Stream Codex rollout JSONL from a fixed file snapshot instead of reading the complete file into one string. The importer now supports valid rollouts beyond the previous 64 MiB cap while bounding source bytes, individual records, converted bytes, and message count.

Source read or conversion failures are reported as a definite source_unreadable result, while failures after persistence begins remain commit_outcome_unknown. Desktop shows the source-specific failure without starting uncertain-commit recovery.

Fixes #4642

Verification

• npm run build:test
• node --test packages/storage/dist/__tests__/codex-session-adapter.test.js packages/runtime-host/dist/__tests__/external-session-coordinator.test.js packages/runtime-host/dist/__tests__/protocol.test.js apps/desktop/dist/main/__tests__/runtime-host-external-sessions-ipc-main.test.js apps/desktop/dist/main/__tests__/import-tasks-settings-page.test.js — 142 passed
• npm run lint
• npm run format:check
• npm run typecheck

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex assisted with diagnosis, implementation, tests, resource-bound ablation experiments, and PR preparation. The commit retains its Generated-by: OpenAI Codex trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No