fix(compaction): reserve full output budget

[dannyward630]

Issue for this PR

Closes#32656

Type of change

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

What does this PR do?

For models with limit.input, compaction reserved at most 20K tokens for the next response even when the model output budget was larger. This PR reserves the full configured/model output budget and adds a boundary test for a 200K input / 32K output model.

How did you verify your code works?

• npx --yes bun@1.3.14 --cwd packages/opencode test test/session/compaction.test.ts

Screenshots / recordings

Not a UI change.

Checklist

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

[github-actions]

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[github-actions]

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