feat(snowflake): use maximum VARCHAR length (128 MB) for string columns

[JGustavo0]

Snowflake's bare STRING/VARCHAR declaration defaults to 16 MB even on accounts where the larger-size behavior change bundles are enabled. This silently caps PeerDB-managed string columns (Postgres text/varchar/bpchar) at 16 MB instead of Snowflake's documented maximum of 128 MB (VARCHAR(134217728)).

This PR adds a shared SnowflakeMaxVarchar constant and uses it for the generic string kind (QValueKindString) and the unknown-kind fallback. The change propagates automatically through qvalue.ToDWHColumnType to:

• generateCreateTableSQLForNormalizedTable — CREATE TABLE
• ReplayTableSchemaDeltas — ALTER TABLE ADD COLUMN
• generateMergeStmt — CAST(... AS <type>) in MERGE

QValueKindEnum (tiny labels) and QValueKindUUID (36 chars) intentionally stay as bare STRING — they can't exceed 16 MB anyway, so widening them would be noise.

Security & performance

• Storage: None. Snowflake bills based on actual data stored; declared length has no storage impact.

• Performance: None. Per the Snowflake docs on string data types:

  When choosing the maximum length for a VARCHAR column, consider that storing a smaller value (e.g. 'sample text') in a column with a larger declared length (e.g. VARCHAR(16777216)) has no impact on performance or storage. There is no performance difference between using the full-length VARCHAR(16777216) declaration and a smaller length.

  The same guarantee extends to VARCHAR(134217728) — declared length is purely a logical ceiling and does not affect compression, micro-partition layout, or query performance.

• Security: Purely a column-width ceiling. Snowflake still enforces the 128 MB hard cap.

Compatibility

The 128 MB ceiling shipped via Snowflake behavior change bundles BCR-1779 (2024_08) and BCR-1942 (2025_03). On accounts where those bundles are disabled, VARCHAR(134217728) DDL will be rejected. Both bundles are enabled by default on current accounts, so this is safe in practice.

Test plan

• flow/shared/types/... unit tests pass (used my account).
• Snowflake schema-delta e2e suite (TestSnowflakeSchemaDeltaTestSuite) runs against a real Snowflake account and passes. Logs confirm string columns are now emitted as VARCHAR(134217728) via the ALTER TABLE ADD COLUMN path.
• Non-string types (BOOLEAN, VARIANT, DATE, TIMESTAMP_*, NUMERIC, FLOAT, BINARY, TIME) emit unchanged.
• BCR-1779/BCR-1942 confirmed enabled on the test account (validated with a manual CREATE TABLE ... VARCHAR(134217728) smoke test).

[JGustavo0]

Anyone could help unlock this?

[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.