Problem

When a ClientSession completes the MCP initialize handshake, the server returns serverInfo (name, version) inside InitializeResult. This data was returned from initialize() but not retained on the session object — callers had no way to inspect which server they were connected to without capturing it at call time.

Fixes #1018

Solution

Add a _server_info: types.Implementation | None attribute to ClientSession, populated during initialize() from result.server_info. Expose it via a get_server_info() method, mirroring the existing get_server_capabilities() pattern:

• Returns None before initialization
• Returns the Implementation (name + version) after a successful handshake

Example

session = await exit_stack.enter_async_context(ClientSession(read, write))
await session.initialize()

server_info = session.get_server_info()
print(server_info.name)     # e.g. "my-mcp-server"
print(server_info.version)  # e.g. "1.0.0"

Changes

• src/mcp/client/session.py: added _server_info field in __init__, assigned in initialize(), exposed via get_server_info()
• tests/client/test_session.py: added test_get_server_info verifying pre-init returns None and post-init returns the correct Implementation