[common] Fix inferred variant shredding failing on decimals with trailing zeros

[zhuxiangyi]

Purpose

This is a bug fix. With variant.inferShreddingSchema=true, writing a Variant that contains a decimal with trailing zeros fails the whole file:

CREATE TABLE T (id INT, v VARIANT) TBLPROPERTIES ('variant.inferShreddingSchema' = 'true');
INSERT INTO T VALUES (1, parse_json('{"price":10.0}')), (2, parse_json('{"price":20.5}'));
-- Job aborted ... java.lang.IllegalArgumentException: Decimal scale must be between 0 and the precision 1 (both inclusive).
--   at org.apache.paimon.types.DataTypes.DECIMAL(DataTypes.java:115)
--   at org.apache.paimon.data.variant.InferVariantShreddingSchema.schemaOf(InferVariantShreddingSchema.java:367)

InferVariantShreddingSchema.schemaOf derives the shredded decimal type from getDecimal(), which strips trailing zeros: 10.0 arrives as 1E+1 and 100.00 as 1E+2, both with a negative scale that DecimalType rejects. The existing guard only covered precision < scale (values below 0.1). Integers beyond the long range that end in zeros (100000000000000000000 → 1E+20) hit the same path. Any engine that writes through the inference path (Spark, Flink) is affected, and 10.0-style values are common in real data.

The fix folds a negative scale back into the digits (setScale(0)) before building the type, the same normalisation VariantGet gained in #9672, and keeps the precision at least the scale. 10.0 now infers as DECIMAL(2, 0), merges with 20.5 into DECIMAL(18, 1), and a lone 100.00 widens to BIGINT as an integer-like decimal, which the shredding writer accepts because it compares against the original scale and allows exact rescaling.

Tests

The new tests reproduce the bug: on master they fail with the IllegalArgumentException above (both the unit test and the two inferred-shredding Spark suites), and pass with this change.

• InferVariantShreddingSchemaTest#testInferSchemaWithDecimalTrailingZeros: 10.0 / 100.00 / 0.05 / 100000000000000000000 infer to DECIMAL(18, 1) / BIGINT / DECIMAL(18, 3) / DECIMAL(38, 0).
• InferVariantShreddingWriteTest#testInferSchemaWithDecimalTrailingZeros: writes such rows with inference on, checks the physical Parquet schema, the reconstructed Variant and typed extraction of $.price / $.whole.
• VariantTestBase: inserts 10.0 / 100.00 and reads them back; runs under all four Spark 4.x configurations. Verified on Spark 4.1.2: 116 tests pass.

API and Format

No.

Documentation

No.