[python] Fix Daft fallback filter limit pushdown

[QuakeWang]

Purpose

The Daft Paimon datasource used the configured read builder for scan planning, but fallback split tasks rebuilt a bare TableRead. As a result, fallback reads for PK merge, non-Parquet formats, BLOB columns, and deletion-vector paths could miss pushed predicate/projection/limit state.

This was correctness-sensitive because Daft may treat pushed filters as already handled by the source. A query filtering a fallback table could plan the right split but still emit unfiltered rows from that split. There was also a related limit-ordering issue: applying limit inside the fallback reader while Daft still had remaining row or partition filters could truncate rows before those filters were evaluated.

This patch makes fallback tasks use a configured TableRead, keeps the required filter columns available for fallback execution, and centralizes the source-side limit decision so limit is only pushed when it is safe for the source to apply it before returning rows.

Tests

python -m py_compile \
  paimon-python/pypaimon/daft/daft_datasource.py \
  paimon-python/pypaimon/tests/daft/daft_data_test.py

python -m pytest paimon-python/pypaimon/tests/daft/daft_data_test.py -q

python -m pytest \
  paimon-python/pypaimon/tests/daft/daft_sink_test.py::TestBlobType::test_write_read_blob_type -q

[XiaoHongbo-Hope]

Looks good to me with little comment.

[JingsongLi]

+1