Giorgi / DuckDB.NET / 38869385553

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Drop the nanoseconds of a timestamp below a tick toward minus infinity

A TIMESTAMP_NS read as DateTime or DateTimeOffset keeps whole ticks of
100 nanoseconds. The rest was dropped toward zero, as DuckDB 1.5 did when
it cast a TIMESTAMP_NS to a TIMESTAMP, so a moment before 1970 moved to a
later time: a nanosecond before 1970 read as 1970-01-01 00:00:00. DuckDB
2.0 rounds toward minus infinity, and the nightly test that compared the
read with its cast failed by one tick.

The digits are now dropped toward minus infinity, as from the digits of
the time of day: a nanosecond before 1970 reads as 1969-12-31
23:59:59.9999999. Values from 1970 on read as before. The test of the
whole range compares with the nanoseconds DuckDB stores (epoch_ns)
rather than with its cast, so it holds for any DuckDB version.

Tested against DuckDB 1.5.6 and v2.0.0-alpha48444: the timestamp test
classes pass on both.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TXdqHuNtM7YXS8B65qdfxX

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