fix(acp): restore session options and reasoning boundaries

[JacobNWolf]

Issue for this PR

Fixes #31961, which was closed but is the same issue as half of this PR.

Type of change

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

What does this PR do?

This PR fixes two bugs:

1. Preserves the ACP reasoning-part boundaries during live streaming and transcript relay. Previously thinking chunks from models like OpenAI's GPT-5.6 Sol were being concatenated with no separators (i.e. "Thinking chunk 1Thinking chunk 2").
2. Restores persisted model, variant, and agent selections when loading, resuming, or forking sessions using ACP. When resuming a session, the model for the session was preselected but its reasoning effort was not picked back up from the session.

How did you verify your code works?

The video below shows me booting a compiled version of OpenCode after the changes in ACP mode in Zed, running a prompt with GPT-5.6 Sol on high effort level, quitting Zed, then reopening and seeing the correct level chosen.

Screenshots / recordings

OpenCodeResumeSession.mp4

Checklist

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

If you do not follow this template your PR will be automatically rejected.

[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.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

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