[Feature Request]: Support correct YYYY-MM-DD formatting when migrating MySQL DATE to Spanner GoogleSQL STRING

### Related Template(s)

sourcedb-to-spanner

### What feature(s) are you requesting?

When migrating MySQL `DATE` columns to Spanner GoogleSQL `STRING(MAX)` in the bulk migration template (`sourcedb-to-spanner`), values are serialized with a full timestamp component (`YYYY-MM-DDTHH:mm:SSZ`, e.g., `2012-09-17T00:00:00Z`) instead of standard date strings (`YYYY-MM-DD`).

As noted in PR #3046, the validation check in `MySQLDataTypesIT.java` (`date_to_string`) was commented out to avoid failing the test suite.

This issue tracks fixing the underlying serialization format for Spanner GoogleSQL databases and re-enabling the test case in `MySQLDataTypesIT`.