Bound state log payloads

[jif-oai]

Why

LogDbLayer is the durable boundary for SQLite diagnostic log storage. Bounding individual call sites is brittle and misses future tracing events.

What Changed

• Cap formatted event fields, span fields, event message, and feedback log bodies to 16 KiB before queueing LogEntry rows.
• Stream DefaultFields formatting through a bounded writer so large Debug/Display fields do not allocate an unbounded formatted string for the log DB path.
• Avoid formatting non-thread_id fields in the thread-id visitors.

Verification

• cargo test -p codex-state log_db
• cargo test -p codex-state
• just fmt
• just fix -p codex-state

[github-actions]

Closing this pull request because it has had no updates for more than 14 days. If you plan to continue working on it, feel free to reopen or open a new PR.