🧪 [Add tests for SVI implied volatility model]

[sharesth23]

🎯 What: The testing gap addressed
This PR addresses the lack of tests for the Stochastic Volatility Inspired (SVI) model in volsurface/models/svi.py. Specifically, it adds unit tests for svi_total_variance and svi_implied_vol functions, ensuring that the model's core logic is tested.

📊 Coverage: What scenarios are now tested
The new test file (volsurface/tests/test_svi.py) covers the following scenarios:

• svi_total_variance: Mathematical correctness with scalar inputs.
• svi_total_variance: Mathematical correctness with 1D numpy array inputs.
• svi_implied_vol: Implied volatility calculation with scalar inputs (computing log moneyness and expected variance).
• svi_implied_vol: Implied volatility calculation with 1D numpy array inputs.
• Input Validation: Ensures that passing a time to maturity T <= 0 raises a ValueError("Time to maturity (T) must be strictly positive.") for both scalar and array inputs.

✨ Result: The improvement in test coverage
The previously untested volsurface/models/svi.py file is now fully tested, improving the reliability of the SVI implied volatility module and mitigating the risk of regressions during future optimizations.

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PR created automatically by Jules for task 621817494366740191 started by @sharesth23

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