DateTime: support calendar units in date arithmetic - #5738

Summary

• extend timestr_to_secs() with an optional start date and support integer calendar years and months alongside existing fixed units
• apply calendar components with end-of-month clamping before fixed-duration components, preserving the original time of day
• use the calendar-aware conversion in Add Time To Date and Subtract Time From Date, including correct backward arithmetic and negative inputs
• document the Robot Framework 7.6 behavior and add unit and acceptance coverage for leap years, year boundaries, mixed units, and invalid values

Validation

• .venv/bin/python utest/run.py — 2403 passed
• .venv/bin/python atest/run.py atest/robot/standard_libraries/datetime — 60 passed
• .venv/bin/invoke library-docs DateTime

Fixes #4890