fix(pool): handle broken connection streams and allow custom server name

[doanhidm1]

Summary of Changes

• Connection Pool (src/custom_connection.py): Check conn._stream_broken instead of accessing _reader directly in SafePool.fill_free_pool() to avoid missing attribute errors on custom connection implementations. Safely close recycled connections with ensure_closed().
• Server Configuration (src/server.py): Support dynamic server naming via the MCP_SERVER_NAME environment variable (default: MariaDB_Server). Updated tool docstrings.
• Tests (src/tests/test_query_validation.py): Added unit test TestSafePool to verify discarding of broken connections in the free pool.