Extract trace duration formatter

[liurenfeng94-ops]

Moves trace duration formatting out of the TreeNode template.

The template had the milliseconds conversion inline, which made the ?? fallback and /1000 expression easy to misread. This keeps the existing displayed value but puts the formatting in a small helper with coverage for defined and missing durations.

Validation:

• pnpm vitest run test/trace-duration-format.test.ts
• pnpm vitest run
• pnpm typecheck
• pnpm exec eslint .
• pnpm build

[github-actions]

We've flagged this as a potential contribution without a human behind it. We welcome the thoughtful use of AI tools when contributing, but ask all contributors to follow two core principles:

1. Never let an LLM speak for you - all comments, issues, and PR descriptions should be written in your own words, reflecting your own understanding.
2. Never let an LLM think for you - only submit contributions you fully understand and can explain.

Please review these AI-assisted contribution guidelines and update this contribution if needed.

If this was flagged in error, we apologise! 😳 Just let us know. 🙏

[liurenfeng94-ops]

Thanks — I reviewed this again. This is a small extraction of the duration formatter out of the template so the milliseconds conversion is explicit and covered by tests.