Fix trace file deletion paths

[liurenfeng94-ops]

Fixes a small path bug in trace cleanup.

When deleting all trace JSON files, the code read entries from the trace directory but called statSync(file) on the bare file name. That only works if the current process happens to be in the trace directory. This changes cleanup to build absolute paths before checking/deleting files.

The path selection is split into a tiny helper so it can be tested without starting VS Code.

Validation:

• pnpm vitest run
• pnpm typecheck
• pnpm exec eslint .
• pnpm build

[github-actions]

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[liurenfeng94-ops]

Thanks for the heads-up — I rewrote the description in my own words. The change is just the cleanup path fix plus a small helper/test split so the path behavior is covered cleanly.