Test Ignite handling of decimal type with negative scale

[zheliu2]

Description

Add test coverage for Ignite connector handling of decimal columns with negative scale.

IgniteClient.toColumnMapping has code at line 253 that claims to support mapping decimal(p, -s) to decimal(p+s, 0), but this code path is untested.

This PR adds testDecimalNegativeScale() to TestIgniteTypeMapping following the same pattern established in #28414 for the PostgreSQL connector.

Additional context and related issues

• Fixes Test Ignite handling of decimal type with negative scale #28416
• See Trino fails when PostgreSQL table has a decimal/numeric column with negative scale #28388 for the original PostgreSQL negative scale issue
• See Fix PostgreSQL connector handling of extreme decimal scales #28414 for the PostgreSQL fix and test pattern

Release notes

( ) This is not user-visible or docs-only

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