chore(add-test): Rename testgen extra to add-test and update Makefile

[karangattu]

This pull request updates the testing dependencies and configuration for the project, primarily by renaming and expanding the test dependency group and correcting a model name in the test generation code. The most important changes are:

• Renamed the testgen dependency group to add-test in pyproject.toml and expanded it to include pytest, pytest-playwright, and playwright for improved test capabilities.
• Updated the Makefile to use the new add-test dependency group instead of the old testgen group for installing test dependencies.
• Fixed the default OpenAI model name in shiny/pytest/_generate/_main.py from gpt-5-mini-2025-08-07 to gpt-5-2025-08-07 to ensure correct model usage.

[github-actions]

Test Generation Evaluation Results (Averaged across 3 attempts)

🔍 Inspect AI Test Quality Evaluation

• Complete (C): 8.3
• Partial (P): 0.3
• Incomplete (I): 0.3
• Passing Rate: 8.6/9.0 (96.3%)
• Quality Gate: ✅ PASSED (≥80% required)

🎯 Overall Result

✅ PASSED - Quality gate based on Inspect AI results

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Results are averaged across 3 evaluation attempts for improved reliability.