fix: skip empty reasoning steps in replayed history

[mihneaptu]

Issue for this PR

Fixes #37651

Context: #37552 documented two independent 400 paths for Kimi K3 on OpenCode Go. The temperature path was fixed in models.dev#3331; this PR fixes the remaining history-replay path, now tracked in #37651.

Type of change

• Bug fix

What does this PR do?

Skips metadata-free empty reasoning parts while rebuilding model history. Some providers can open and close a reasoning stream without emitting a delta; replaying that boundary previously produced an empty assistant message before the next step.

How did you verify your code works?

• Added a regression test for the duplicate step-start / empty-reasoning sequence.
• bun test test/session/message-v2.test.ts: 37 passed.
• Replayed the affected Kimi K3 max session through the patched CLI; it continued for 26 steps and finished normally.

Screenshots / recordings

Not applicable.

Checklist

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

[github-actions]

Hey! Your PR title Skip empty reasoning steps in replayed history doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.