fix: case-insensitive Snowflake relation matching for catalog lookup in get_columns

[Felix313]

Summary

Fixes Snowflake catalog lookup failures caused by case-sensitive relation matching in get_columns().

Problem

When target/catalog.json contains uppercase relation names (common in Snowflake), dbt-osmosis may look up the same relation in lowercase and thus fails to match it.

Example mismatch:

• searched: db.schema.table_name
• catalog: db.schema.TABLE_NAME

This prevents selecting the catalog entry and can lead to ApproximateMatchError / missing metadata behavior.

Root Cause

Catalog matching used:

• relation.matches(*c.key())
  which is case-sensitive for this Snowflake scenario.

Change

In src/dbt_osmosis/core/introspection.py (get_columns()):

• Added Snowflake-specific case-insensitive catalog matching by normalizing both sides to uppercase before comparison.
• Kept existing behavior for non-Snowflake adapters.
• Added safe fallback to default matcher if normalization path fails.

Validation

Validated locally with:
`dbt-osmosis yaml refactor TABLE_NAME --catalog-path target\catalog.json

Result: catalog entry is found correctly despite case differences.

Related / Complementary

This PR is complementary to #322 .

• fix(transforms): use case-insensitive column comparison when output case settings are active #322 addresses column-level case handling in transforms.
• This PR addresses relation-level case handling during catalog lookup in get_columns().