fix(opencode): clone part data in Bus event to preserve token values

[ryanskidmore]

Issue for this PR

Closes #15779

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Session.updatePart() passes the part object by reference into Bus.publish(PartUpdated) inside a Database.effect(). The fn() wrapper's Zod schema.parse() creates a new object from the input, and the deferred effect closure captures the original reference. If anything mutates that object between queue time and effect execution, JSONL subscribers receive stale data with zeroed token fields — while the TUI is unaffected since it reads from the DB post-persistence.

The bug is timing/provider-dependent: Database.effect() fires synchronously after Database.use() returns, so in most cases the reference is still valid. It manifests under specific async timing conditions depending on the provider.

The fix wraps the part in structuredClone(part) to snapshot the values at queue time, guaranteeing isolation regardless of timing.

How did you verify your code works?

• bunx tsc --noEmit passes
• bun test test/session/session.test.ts — 3 pass, 0 fail (includes new regression test with 10 assertions verifying non-zero tokens propagate through the Bus event)
• Regression test also confirms the event part is a clone, not the same object reference
• Verified bun dev --format json run produces non-zero tokens in step_finish JSONL output

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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