[BugFix] Fix pre-1970 Parquet timestamp load corrupting sub-second DATETIME (backport #75207)

[mergify]

Why I'm doing:

When loading a Parquet INT64 column annotated TIMESTAMP (isAdjustedToUTC=false) into a StarRocks DATETIME, a pre-1970 value with a nonzero sub-second part was decoded to a corrupt garbage value instead of the real wall clock.

Int64ToDateTimeConverter::convert splits the signed epoch tick with C++ truncating division:

int64_t seconds     = src_data[i] / _second_mask;
int64_t nanoseconds = (src_data[i] % _second_mask) * _scale_to_nano_factor;

For a negative tick whose sub-second remainder is nonzero, nanoseconds is negative. timestamp::of_epoch_second then packs the result via a bitwise OR (from_julian_and_time), so the negative microsecond corrupts the packed Julian field — e.g. 1969-12-31 23:59:59.500 loaded as a year-41222 garbage value. Whole-second negatives and all post-1970 values were unaffected.

What I'm doing:

Borrow a whole second when the sub-second remainder is negative, so nanoseconds stays in [0, NANOSECS_PER_SEC) — the floor split the FE boundary computation already uses (Math.floorDiv/Math.floorMod). of_epoch_second then receives a non-negative sub-second and packs the correct value. The borrow is unit-agnostic (MILLIS/MICROS/NANOS) and runs before the UTC whole-second offset, so it composes with the timezone-adjusted branch unchanged.

Added a regression test (Int64PreEpochTimestampSubSecond) that loads a Parquet file holding 1969-12-31 23:59:59.500000 in both MILLIS and MICROS units: it decoded to a garbage value before the fix and to the correct wall clock after. The existing post-1970 Int64_2_Timestamp test is unchanged (14/14 ColumnConverterTest pass).

What type of PR is this:

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Does this PR entail a change in behavior?

• Yes, this PR will result in a change in behavior.
• No, this PR will not result in a change in behavior.

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr needs user documentation (for new or modified features or behaviors)
  • I have added documentation for my new feature or new function
  • This pr needs auto generate documentation
• This is a backport pr

Bugfix cherry-pick branch check:

• I have checked the version labels which the pr will be auto-backported to the target branch
  • 4.1
  • 4.0
  • 3.5

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    This is an automatic backport of pull request [BugFix] Fix pre-1970 Parquet timestamp load corrupting sub-second DATETIME #75207 done by Mergify.