refactor: separate performance_metrics into metrics module to resolve circular import

[fabiooliveir]

Summary

This PR addresses the TODO in prophet/plot.py:

# TODO: separate performance_metrics into a different module. there is an implicit circular import between forecaster.py and diagnostics.py

Changes

• Created prophet/metrics.py containing all pure evaluation metrics (mse, rmse, mae, mape, mdape, smape, coverage), rolling aggregation helpers (rolling_mean_by_h, rolling_median_by_h), and performance_metrics.
• Updated prophet/plot.py to import performance_metrics from prophet.metrics instead of prophet.diagnostics.
• Re-exported all metrics and helper symbols in prophet/diagnostics.py to maintain 100% backward compatibility for existing code.
• Added dedicated unit tests in prophet/tests/test_metrics.py (19 test cases covering functionality and backward compatibility).

Test Plan

• python -m pytest prophet/tests/test_metrics.py passed (19/19 passed).
• python -m pytest prophet/tests/test_diagnostics.py passed (26/26 passed).
• Static type check (pyrefly check) passed with 0 errors on prophet/metrics.py.

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