[python] Expand native read coverage for nested and streaming reads

[JingsongLi]

Purpose

Use the Rust reader for more PyPaimon output paths instead of falling back to Python for nested projection, row kinds, and continuous streaming frames.

Changes

• forward nested name paths and row-kind requests to pypaimon-rust
• flatten native nested ROW and literal MAP-key results with parent-null propagation
• preserve the public _row_kind schema through predicate-only output projection
• share batch ReadBuilder projection semantics with StreamReadBuilder
• use native planning for eligible initial and delta streaming frames, with safe fallback for bucket filters, authorization, snapshot mismatch, and setup errors
• cover Parquet shared-shredding MAP, ORC, Avro, ROW, optional Mosaic, primary-key merge, partial-update, schema rename, Data Evolution, predicate, limit, parallelism, and streaming reads

Depends on apache/paimon-rust#887. pypaimon-rust has not been released, so this intentionally targets the paired API without a legacy compatibility shim.

Tests

• 182 targeted Python tests passed, plus 15 format/parameter subtests
• project flake8 configuration passed on all changed Python files
• paired Rust tests and clippy are recorded in feat: expand Python native reads with nested projection and row kinds paimon-rust#887

[leaves12138]

Reviewed with a freshly built apache/paimon-rust#887 native extension. The six targeted Python suites produced 171 passes and one failure in the new optional Mosaic subcase below. I also reported the separate empty-string MAP-key decoding issue on the paired Rust PR.

[leaves12138]

[P2] Gate the Mosaic subcase on nested-ROW support, not just importability

paimon-mosaic==0.1.0 satisfies the existing optional dependency (paimon-mosaic>=0.1.0 in setup.py) and makes this condition true, but its writer cannot write the nested payload Struct used by this test. With that supported dependency version and the freshly built #887 extension, this test fails while creating the fixture, before it reaches either reader: RuntimeError: writer open failed: unsupported DataType: Struct([Field { name: "details", data_type: Struct(...) }, ...]). The other 171 tests in the targeted suites passed.

Please gate this subcase on a Mosaic version/capability that supports the schema, adapt its fixture to supported types, or explicitly raise the optional dependency minimum if requiring a newer writer is intentional. Otherwise installing an allowed Mosaic version makes the new native-read suite fail for reasons unrelated to the reader being tested.

[leaves12138]

Re-reviewed the update. The Mosaic test now probes nested-ROW write support instead of assuming that an importable package supports the fixture. With paimon-mosaic 0.1.0, the previously failing test now passes by excluding that unsupported subcase; I also checked that unrelated writer errors still propagate. Using a freshly rebuilt apache/paimon-rust#887 extension at 304703c6771c6389147823f51afed66dc7e8337d, all 172 targeted Python tests and four independent regression checks pass. The changed file passes the project flake8 configuration. No remaining blocking findings. LGTM.