[zhang-arvin]

What problem does this PR solve?

Fixes #68802.

Reading a Paimon primary-key table whose ARRAY<ROW> column uses the nested_update aggregate function fails through the JNI scanner with a ClassCastException (e.g. HeapBytesVector cannot be cast to LongColumnVector) on splits that need merge-on-read. It only reproduces for such splits, so it is intermittent and disappears once files are compacted; SET enable_prune_nested_column = false masks it.

Root cause: paimon's merge engine (FieldNestedUpdateAgg) builds its key projection against the column's declared element row type and reads the nested-key field by its original position. Doris' nested-column pruning narrows the read type to the sub-fields the query touches and pushes it down with ReadBuilder.withReadType; when the nested-key (the first element field) is dropped, the merge engine still projects that position out of the narrower rows and reads whatever now sits there.

What changed

• New PaimonNestedUpdateColumns: resolves, from the table's merged options, every column with fields.<col>.aggregate-function = nested_update that also declares a fields.<col>.nested-key.
• PaimonJniScanner.initReader now keeps the whole element row (instead of the pruned shape) for such a column, so the merge engine's positional projection stays valid. The query still reads only the fields it asked for; the extra fields exist only to satisfy the merge engine.
• Non-nested_update ARRAY<ROW> columns keep ordinary nested-column pruning.

Tests

• PaimonReadTypeProjectionTest: a nested_update column keeps its whole element row when pruned; a plain ARRAY<ROW> column still narrows.
• PaimonNestedUpdateColumnsTest: option resolution (single/multi field nested-key, case-insensitivity, a nested_update column without a nested-key, non-primary-key tables).
• Verified the new projection test fails without the fix.

mvn -pl be-java-extensions/paimon-scanner -am test: 117 tests pass, checkstyle clean.

[hello-stephen]

Thank you for your contribution to Apache Doris.
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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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