fix(llm): tolerate wrapped workflow tool args

[sudorest]

Issue for this PR

Closes #20757

Type of change

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

What does this PR do?

This fixes a workflow edit-writing failure where GitLab workflow tool calls could surface unterminated string JSON parse errors before the tool ran.

The workflow executor was calling JSON.parse directly on the raw tool args string. For edit flows, some model outputs can wrap the object payload in fenced JSON or surrounding prose. This change parses object-shaped tool args more defensively in that workflow path, still rejecting malformed or non-object inputs with a clear error.

How did you verify your code works?

I added focused regression coverage in packages/opencode/test/session/llm.test.ts for:

• strict JSON tool args
• fenced JSON tool args
• prose-wrapped JSON object args
• malformed unterminated-string args
• non-object JSON args

The original broader workflow-path test was replaced with direct parser coverage because the integration version relied on global monkey-patching and a fake workflow model shape, which was brittle in CI.

Screenshots / recordings

Not a UI change.

Checklist

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

[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

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[github-actions]

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

[rekram1-node]

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