fix(opencode): preserve valid empty JSON config

[aditya-vithaldas]

Issue for this PR

Closes #36374

Type of change

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

What does this PR do?

Avoids adding a dangling comma when $schema is inserted into an empty config object. The comma is now included only when the parsed config already has keys, so {} remains valid strict JSON after being rewritten.

How did you verify your code works?

• bun test test/config/config.test.ts (97 passed)
• bun typecheck from packages/opencode
• Added a regression test that parses the rewritten file with JSON.parse

Screenshots / recordings

Not a UI change.

Checklist

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

[github-actions]

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

Potential duplicate found:

• PR fix(config): write valid JSON when auto-adding $schema to an empty config #36380: fix(config): write valid JSON when auto-adding $schema to an empty config
  • fix(config): write valid JSON when auto-adding $schema to an empty config #36380

This appears to be addressing the exact same issue as PR #36468. Both PRs are fixing the problem of writing invalid JSON when $schema is automatically added to an empty config object (the dangling comma issue). You should review PR #36380 to determine if it's already solving this problem, or if these represent different approaches to the same fix.

[github-actions]

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