feat: store server_info on ClientSession after initialization

[kxbnb]

Summary
Stores server_info from the init response on ClientSession so you can grab the server's name and version without manually extracting from the result.

Changes

• Added _server_info field to ClientSession.__init__
• Store result.server_info during initialize()
• Added get_server_info() getter (mirrors existing get_server_capabilities())
• Added test for the new method

Before

server = await session.initialize()
name = server.serverInfo.name  # had to grab manually

After

await session.initialize()
info = session.get_server_info()
name = info.name  # available directly

Testing

• All existing tests pass
• Added test_get_server_info covering None-before-init and populated-after

Closes #1018