feat: Ignore temporary schemas ending with _next or _NEXT

[lpillmann]

Summary

Fixes intermittent errors when temporary Snowflake schemas with _next suffix disappear during permifrost execution. Our ingestion framework creates temporary schemas with _next suffix (lower or upper case) that may be removed between when permifrost starts inspection and when it executes.

Problem

When permifrost starts, it sees these temporary schemas and includes them in its execution plan. If the schema disappears before permifrost finishes (e.g., the ingestion run completes), permifrost errors with object not exists or similar errors.

Solution

Filter out schemas ending with _next (case-insensitive) in show_schemas() — the central method called whenever permifrost needs to enumerate schemas in a database. This prevents temporary schemas from ever entering the execution plan.

Changes

• src/permifrost/snowflake_connector.py — Added filtering in show_schemas() to skip schemas ending with _NEXT (case-insensitive check via .upper().endswith("_NEXT"))
• tests/permifrost/test_snowflake_connector.py — Added test_show_schemas_filters_next_suffix() covering _next, _NEXT, and _Next variants; also fixed missing snowflake_connector_env fixture on test_show_schemas

Testing

• 20 unit tests passing (all existing + 1 new)
• New test verifies uppercase, lowercase, and mixed-case _next suffixes are all filtered
• Verified with flake8, black, and mypy — all clean

Related

• Linear: GEM-5