feat: Integrate OpenRouterModel into Metric utilities

[Djalal-H]

PR: Add OpenRouter support to Metrics

This PR integrates the OpenRouterModel into the deepeval metrics layer, allowing it to be used as a native evaluation provider.

Key Changes

• deepeval/metrics/utils.py:
  • Added OpenRouterModel to recognized native models.
  • Implemented should_use_openrouter_model() for automated provider detection.
  • Updated initialize_model() and is_native_model() to support OpenRouter.
• Tests: Added tests/test_integrations/test_openrouter/test_g_eval_openrouter.py to verify GEval integration, including sync/async execution and cost tracking.

Testing

Run pytest tests/test_integrations/test_openrouter/test_g_eval_openrouter.py with OPENROUTER_API_KEY configured.

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