Support Paimon database statements in DataFusion SQLContext

[shyjsarah]

Problem

SQLContext delegates database statements to DataFusion, whose database semantics do not match Paimon:

• CREATE DATABASE creates a DataFusion catalog instead of a Paimon database.
• SHOW DATABASES, DROP DATABASE, and USE <database> are not supported.
• Paimon databases are exposed internally as DataFusion schemas, so users currently need schema terminology for database operations.

Proposed behavior

Handle the following statements directly through the current Paimon Catalog:

SHOW DATABASES;
CREATE DATABASE [IF NOT EXISTS] [catalog.]db;
DROP DATABASE [IF EXISTS] [catalog.]db [CASCADE];
USE [catalog.]db;

Keep CREATE SCHEMA and DROP SCHEMA working as compatibility aliases. Unsupported database statement options should fail explicitly rather than being ignored.