Handle omitted Act/365L reference end date

[kadyrbekovhamit-cyber]

Summary

DayCount.year_frac() documents dt3 as optional for an ordinary two-date year-fraction calculation. In the annual ACT_365L branch, however, a period that reaches a leap day compares that leap day with dt3 even when dt3 is None. The call then raises AttributeError instead of returning a fraction.

This change treats dt2 as the calculation-period end when no separate reference-period end is supplied. A caller that supplies dt3 for an accrued fraction keeps the existing reference-period behaviour.

Reproducer

DayCount(DayCountTypes.ACT_365L).year_frac(
    Date(1, 12, 2023),
    Date(1, 3, 2024),
    freq_type=FrequencyTypes.ANNUAL,
)

Before this change:

AttributeError: 'NoneType' object has no attribute 'excel_dt'

After this change:

(0.24863387978142076, 91.0, 366)

The regression also distinguishes the ordinary two-date mode from an accrued-fraction call with an explicitly later coupon-period end, so the correction does not erase the existing dt3 semantics.

Validation

• python -m pytest unit_tests/test_FinDayCount.py -q: 11 passed
• python -m pytest unit_tests -q: 959 passed, with four pre-existing LSMC RankWarning messages
• Restoring the previous None handling reproduces the exception above.

The example is synthetic. This PR makes no claim about a downstream system or financial loss.