fix: ensure database context is set before schema creation in Snowflake

[devin-ai-integration]

Fixes Snowflake schema creation by ensuring database context is properly set.

This PR:

1. Overrides _ensure_schema_exists in SnowflakeSqlProcessor to properly set database context
2. Uses fully qualified schema names in CREATE SCHEMA statements
3. Maintains existing error handling for schema existence

Link to Devin run: https://app.devin.ai/sessions/be3168a4b7974151b7cf9f082e47e893

[devin-ai-integration]

Original prompt:

@Devin - PyAirbyte CI tests are failing in new PRs. Can you investigate. Looks like Snowflake account is missing a DB or has its default DB changed.

[devin-ai-integration]

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[Aaron ("AJ") Steers (aaronsteers)]

Replaced by:

• fix: Poetry builds and snowflake tests #595