fix(task): surface non-text subagent results

[smithyyang]

Issue for this PR

Closes #24447

Type of change

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

What does this PR do?

This fixes the task tool returning an empty <task_result> when a subagent completes without a final text part.

Before this change, TaskTool only looked for the last text part and fell back to "" if none existed. That made reasoning-only or non-text child completions look like blank results in the parent session.

This change keeps the existing behavior when a child returns text, and otherwise returns a short diagnostic summary with the finish reason, error, and emitted part types.

How did you verify your code works?

• Ran bun run typecheck in packages/opencode
• Ran bun test test/tool/task.test.ts --test-name-pattern "execute returns child text output when present|execute returns diagnostic output when child finishes without a text part"
• Ran bun test test/cli/run/entry.body.test.ts test/cli/run/scrollback.surface.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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2. Add Fixes #<number> or Closes #<number> to this PR description

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