add session-persistent tools context manager

[zndqs]

Description

This PR introduces a new tools() async context manager method to MultiServerMCPClient, providing persistent session support when working with MCP tools. The implementation:

Adds @asynccontextmanager tools() method that maintains active connections

Supports both single-server and all-servers tool loading patterns

Includes comprehensive documentation with usage examples

Maintains backward compatibility with existing get_tools()

Motivation

Currently, get_tools() creates a new session for each invocation, which can be inefficient for stateful tool usage. The new context manager approach:

Keeps connections alive during tool operations

Enables better resource management

Provides more intuitive session lifecycle control

Matches patterns used elsewhere in the codebase

Usage Examples

# Single server usage
async with client.tools(server_name="math") as tools:
    # Tools remain connected
    result = await tools[0].arun(...)

# All servers usage 
async with client.tools() as all_tools:
    # Work with combined tools from all servers

Related Issues

Closes #207 (if applicable)

[l2leonardo]

Does this keep the session alive outside the with block?

[zndqs]

Does this keep the session alive outside the with block?

no, you can try use xxx.tool().aenter and aexit separatly

[l2leonardo]

@zndqs I don't think this actually fix the issue. Session is not persistent on the agent as a whole, only if you hit a Tool Calling node and have a stream of tools already prepared for calling. I think the current version of adapters already allows you to do something like this if you add a with block in your tool calling node...

[eyurtsev]

stateful connections in MCP do not only involve tools but can invole resources etc.

A context manager that opens a stateful connection to a server makes sense, but it shouldn't be limited to only tools.