[core] Fix VARIANT timestamp projection and scalar conversions

[XiaoHongbo-Hope]

Purpose

Fix incorrect VARIANT timestamp projections, missing Boolean-to-numeric casts, and decimal shredding overflow.

Brief change log

• Convert timestamp microseconds to the requested Arrow unit; nanosecond overflow errors in strict mode and returns NULL in tolerant mode.
• Support Boolean-to-integer, DOUBLE and DECIMAL extraction, consistent with Java.
• Keep integers in residual storage when they cannot fit the shredded DECIMAL column.

Tests

Parquet round-trip regressions cover plain/shredded VARIANT, timestamp units and overflow, Boolean casts, and decimal residuals.

• cargo test -p paimon --lib variant: 93 passed.
• cargo test -p paimon --lib arrow::shredding: 49 passed (overlapping coverage).
• cargo fmt --all -- --check and git diff --check passed.

API and Format

No public API or storage format changes.

Documentation

Not required for these correctness fixes.

[JingsongLi]

+1