fix(cdc): normalize zero length varchar metadata

[hutiefang76]

Purpose

Fixes #4556.

When CDC metadata contains a string-like column with length 0 (for example from a MySQL ENUM column), DataTypeConverter#getLogicalType passed that value directly to Flink's VarCharType. Flink rejects VARCHAR(0) and throws:

Variable character string length must be between 1 and 2147483647

Changes

• Normalize null or invalid varchar lengths to VarCharType.MAX_LENGTH before creating the Flink logical type.
• Add a regression test for a zero-length varchar column mapped through DataTypeConverter.

Verification

Ran locally with JDK 17:

./mvnw -Pflink-single-version,flink-1.18 \
  -pl dinky-flink,dinky-client,dinky-cdc/dinky-cdc-core \
  -am -Dtest=DataTypeConverterTest -DfailIfNoTests=false test

Result: build success, DataTypeConverterTest passed.