test: cover Mosaic read compatibility

[QuakeWang]

Purpose

Linked issue: close #378

Mosaic read support is already implemented and covered by Rust-generated reader tests, but the compatibility claim was not validated with a Paimon table fixture generated outside the current Rust test path. The docs also did not describe how to enable Mosaic reads or the current read-only scope.

Brief change log

• Add a small PyPaimon-generated Mosaic table fixture.
• Add a DataFusion read-only compatibility test covering full reads, projection reorder, SQL filtering, and fixture .mosaic presence.
• Document the mosaic feature and current Mosaic read-only limitations.

Tests

• cargo fmt --all -- --check
• cargo test -p paimon-datafusion --features mosaic --test mosaic_tables
• cargo clippy -p paimon-datafusion --features mosaic --test mosaic_tables -- -D warnings
• cargo check -p paimon-datafusion --features mosaic --tests

API and Format

Documentation

Added Mosaic feature usage and read scope documentation.

[JingsongLi]

+1