Bugs in simple-streamablehttp-stateless

[yangyang-yyy]

Describe the bug
There are two bugs in servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless, main branch.

• mcp.server.streamableHttp has been renamed
• The method 'run' in class 'Server' in lowlevel/server.py doesn't have 'standalone' attribute.

[ihrpr]

thank you! should be fixed in #641 - merging soon