docs(sql): Expand docstring for get_primary_keys to explain fallback behavior

- Add explanation that method uses primary_key if set explicitly in configured catalog
- Otherwise falls back to source_defined_primary_key if set
- Addresses GitHub comment from @aaronsteers

Co-Authored-By: AJ Steers <[email protected]>

Author: devin-ai-integration[bot]

airbyte_cdk/sql/shared/catalog_providers.py

@@ -120,7 +120,11 @@ def get_primary_keys(
         self,
         stream_name: str,
     ) -> list[str]:
-        """Return the primary keys for the given stream."""
+        """Return the primary keys for the given stream.
+        
+        We will use `primary_key` if it is set explicitly in the configured catalog, 
+        otherwise we will fall back to `source_defined_primary_key`, if set.
+        """
         configured_stream = self.get_configured_stream_info(stream_name)
         pks = (
             configured_stream.primary_key