[common] Read a shredded variant decimal as the same string as an unshredded one

[zhuxiangyi]

Purpose

This is a bug fix. Extracting a decimal as a string returns different text depending on whether the file is shredded:

CREATE TABLE T (id INT, v VARIANT);
INSERT INTO T VALUES (1, parse_json('{"price":1.50}')), (2, parse_json('{"price":0.05}'));

SELECT variant_get(v, '$.price', 'string') FROM T;
-- plain file, or Spark evaluating itself : "1.5",  "0.05"
-- file shredded with price DECIMAL(18, 2): "1.50", "0.05"

The unshredded leg (VariantGet.cast) casts from getDecimal(), which strips trailing zeros, so it matches Spark's VariantGet. The shredded leg (BaseVariantReader.ScalarReader) reads the typed_value with the scale of the file schema and its string cast keeps that scale. Only decimal → string is affected; numeric targets agree on both legs.

This matters because the docs promise that "the SQL and result are the same for plain, shredded, and mixed-layout files", 'string' is the one target type a strict variant_get is pushed into the scan for, and with variant.inferShreddingSchema a table routinely mixes plain and shredded files, so a single column yields 1.5 for some rows and 1.50 for others.

The fix extracts the normalization already used by the unshredded leg (strip trailing zeros, fold a negative scale, keep precision at least the scale) into VariantGet.normalizedDecimal and applies it to a typed_value decimal before it is cast, so both legs cast from the same value.

Tests

The new unit and format tests reproduce the bug: on master they fail with expected: +I(10,1.5,0.05,0,...) but was: +I(10.0,1.50,0.05,0.00,...), and pass with this change.

• PaimonShreddingUtilsTest#testShreddedDecimalCastsLikeUnshredded: the same extractions through a shredded and an unshredded VariantSchema yield identical strings, and decimal / double / bigint targets are unchanged.
• VariantShreddingReadTest#testReadDecimalAsStringConsistently: parameterised over a plain Parquet file and one shredded with price DECIMAL(18, 1), amount DECIMAL(18, 2).
• VariantTestBase: variant_get(..., 'string') on values whose inferred scale keeps trailing zeros; runs under all four Spark 4.x configurations (with/without pushdown, with/without inferred shredding). Verified on Spark 4.1.2: 116 tests pass.

API and Format

No.

Documentation

No.