fix: avoid NOT NULL in Snowflake merge casts

[jamesg31]

Summary

• generate Snowflake MERGE cast types without column nullability constraints
• preserve NOT NULL generation for destination DDL types
• add a regression test covering required and nullable timestamp columns

Context

PR #3893 fixed the same Snowflake syntax failure by keeping nullability out of CAST types. PR #3923 later routed TableSchema.NullableEnabled into ToDWHColumnType from the MERGE generator, which can emit invalid expressions such as CAST(... AS TIMESTAMP_TZ NOT NULL).

The Snowflake connector is deprecated, but this is a focused regression repair for existing deployments.

Test

go test ./connectors/snowflake -run '^TestGenerateMergeStatementNullableSchemaDoesNotAddNotNullToCasts$'

[CLAassistant]

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[github-actions]

⚠️ Deprecated destination connector

This issue or pull request relates to a deprecated destination connector (Snowflake, BigQuery, ElasticSearch, Kafka, Redpanda, Confluent, Azure Event Hubs, Google Pub/Sub, or S3).

These destinations are no longer actively maintained, but remain functional. We are unlikely to prioritize new work here.

Note: BigQuery is deprecated only as a destination — it remains a supported source.

If you depend on one of these connectors, we recommend:

• Pin to a known-good PeerDB version so behavior stays stable.
• Fork the repository if you need to carry your own changes.

See the deprecated connectors documentation for details and migration guidance.