[flink] Push down lake filters for non-partitioned scans

[luoyuxia]

Purpose

Linked issue: close #3168

Allow FlinkTableSource to push filters into LakeSource for non-partitioned table scans. Before this change, lake filter pushdown was nested under isPartitioned(), so non-partitioned scans in lake-backed FULL reads always fell back to Flink-side filtering for historical data.

Brief change log

• extract lake filter pushdown into a dedicated helper and invoke it whenever lakeSource != null
• preserve the existing fallback to Flink-side filtering when the lake source rejects part of the predicate set
• add unit coverage for non-partitioned lake pushdown and the fallback path

Tests

• ./mvnw -pl fluss-flink/fluss-flink-common -DskipITs test

API and Format

No API or storage format changes.

Documentation

No documentation changes.