test: cover Mosaic read path combinations

[QuakeWang]

Purpose

Linked issue: #378

The Mosaic reader already supports projection, missing physical columns, row selection, deletion vectors, and row-group pruning, but the test coverage did not exercise several important combinations across the format reader and DataFileReader layers. This made regressions around schema alignment, null preservation, stats conversion, and composed read filters easy to miss.

Brief change log

• Add Mosaic format tests for supported scalar/temporal type round-trips and nullable values.
• Cover Mosaic stats value conversion into Paimon Datum, including fail-open cases.
• Cover batch splitting behavior used by Mosaic reads.
• Add DataFileReader tests for Mosaic with deletion vectors, mixed Mosaic/Parquet splits, and predicate + deletion vector + projection composition.

Tests

• cargo fmt --all -- --check
• git diff --check
• cargo test -p paimon --features mosaic mosaic -- --nocapture
• cargo clippy -p paimon --features mosaic --tests -- -D warnings

API and Format

Documentation

[JingsongLi]

+1