feat(table): read materialized deletion-vector PK splits

[shyjsarah]

Purpose

Linked issue: close #579

Allow read-only access to fully materialized partial-update and aggregation primary-key plans with deletion vectors. This change is independent of #573.

Brief change log

• Route eligible plans through DataFileReader so deletion vectors filter deleted rows.
• Require raw-convertible compacted files with known zero delete_row_count and merge-on-read disabled.
• Fail closed for level-0 files, ambiguous metadata, and plans that still require per-key merging.

Tests

• Partial-update and aggregation compacted DV reads.
• Level-0, non-raw-convertible, unknown/nonzero delete-row count, and merge-on-read rejection.
• cargo test -p paimon --lib
• cargo test -p paimon-datafusion --test pk_tables
• cargo clippy -p paimon --lib --tests -- -D warnings

API and Format

No public API or storage format changes.

Documentation

Updated the SQL merge-engine documentation with the supported read-only scope and limitations.