[FIX] Correct probability mass and temporal alignment in _SksurvAdapter #958

[vishalsnghkush]

This PR fixes mathematical errors in the predict_proba method of _SksurvAdapter. Previously, the resulting Empirical distributions were improper (weights summed to less than 1.0) and temporally shifted.

Changes:

1. Initial Mass Preservation: Prepends a survival value of 1.0 to the survival function to capture the first drop from t=0.
2. Tail Mass Preservation: Appended the remaining survival probability at the last time point to np.inf. This ensures the total mass always sums to 1.0.
3. Alignment Correction: Fixed the logic where np.diff results were shifted. Probability drops are now correctly mapped to the timestamps where they occur.

Testing & Verification:
Verified using the reproduction script in Issue 958.

• Before Fix: Total weight was less than 1.0.
• After Fix: Total weight equals 1.0 and includes np.inf for censored mass.