Fix task completion tool prompt rendering

[luochen211]

Summary

• format tools_called before rendering TaskCompletionMetric's extract_goal_and_outcome template
• cover sync and async prompt construction with regression tests

Fixes #2807

Tests

• /tmp/deepeval-pr-venv/bin/python -m pytest tests/test_metrics/test_task_completion_prompt.py
• python3 -m pytest tests/test_templates/test_metric_templates.py

[vercel]

@luochen211 is attempting to deploy a commit to the Confident AI Team on Vercel.

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[luochen211]

GitHub Actions are green after formatting with the current Black version. The only remaining failing status is Vercel, which says a Confident AI team member needs to authorize the deployment.