Add schema introspection and raw SQL support for Dremio adapters

Dremio Adapter Enhancements:
- Add FK introspection via pg_catalog for PostgreSQL sources
- Add table/column comments via pg_description
- Add nested type introspection via LIMIT 0 queries
- Add ADBC Flight SQL driver support (faster queries)
- Fix table path quoting for schemas with hyphens/spaces
- Add _qualify_table_names() for automatic table qualification

New Features:
- Add supports_sql property and execute_sql() to Database base class
- Add --sql flag to CLI query command for raw SQL execution
- Add execute_sql() to DuckDB adapter

The Dremio adapters now support:
- Schema introspection with FK relationships from PostgreSQL sources
- Raw SQL queries via CLI: linkml-store -d dremio://... query --sql '...'
- Automatic table name qualification when schema is configured
- Proper quoting of source names containing hyphens/spaces

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cmungall

src/linkml_store/api/database.py

@@ -441,6 +441,48 @@ def query(self, query: Query, **kwargs) -> QueryResult:
         else:
             raise NotImplementedError(f"Querying without a table is not supported in {self.__class__.__name__}")
 
+    @property
+    def supports_sql(self) -> bool:
+        """
+        Return whether this database supports raw SQL queries.
+
+        Backends like DuckDB, PostgreSQL, Dremio support SQL.
+        Backends like MongoDB, filesystem do not.
+
+        :return: True if raw SQL is supported
+        """
+        return False
+
+    def execute_sql(self, sql: str, **kwargs) -> QueryResult:
+        """
+        Execute a raw SQL query against the database.
+
+        This method allows direct SQL execution on SQL-capable backends,
+        bypassing the linkml-store query abstraction layer.
+
+        :param sql: SQL query string
+        :param kwargs: Additional arguments
+        :return: QueryResult containing the results
+        :raises NotImplementedError: If this backend does not support SQL
+
+        Examples
+        --------
+        >>> from linkml_store.api.client import Client
+        >>> client = Client()
+        >>> db = client.attach_database("duckdb", alias="test", recreate_if_exists=True)
+        >>> collection = db.create_collection("Person")
+        >>> collection.insert([{"id": "P1", "name": "John"}, {"id": "P2", "name": "Alice"}])
+        >>> result = db.execute_sql("SELECT * FROM Person WHERE name = 'John'")
+        >>> len(result.rows)
+        1
+        >>> result.rows[0]["name"]
+        'John'
+        """
+        raise NotImplementedError(
+            f"Raw SQL queries are not supported by {self.__class__.__name__}. "
+            f"Use collection.find() or collection.query() instead."
+        )
+
     @property
     def schema_view(self) -> SchemaView:
         """