feat(opencode): use reasoning summary auto for gpt-5 models that are not chat

[scratchmex]

this mimicks what is already done for opencode/ models

What does this PR do?

Please provide a description of the issue (if there is one), the changes you made to fix it, and why they work. It is expected that you understand why your changes work and if you do not understand why at least say as much so a maintainer knows how much to value the pr.

How did you verify your code works?

using a gpt-5 model with no variant selected. i.e without any low, medium, etc; text

without the change

with the change

fixes #12500

[github-actions]

Hey! Your PR title use reasoning summary auto for gpt-5 models that are not chat 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]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR:

#12391 - "fix: Recursively compute reasoning variants for Vercel AI Gateway provider based on model id"

This PR appears related because it also deals with reasoning variants and model handling, though it focuses on the Vercel AI Gateway provider specifically. It's worth reviewing to ensure there's no overlap with the current PR's changes.

The other results (like #6114 about GPT-5.2 resume reasoning crash and #11572 about stripping reasoning parts) are related to reasoning features but address different specific issues.

[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.