ClientSession: add public API for updating callbacks after initialization

定位 ClientSession，检查现有的 _list_roots_callback、_sampling_callback 和 _elicitation_callback 属性及其使用方式。查看 ClientSession 附近的测试，然后添加并测试一种公开方式来更新每个 callback，使更改影响已经初始化的 session。

Problem

ClientSession accepts callback parameters (list_roots_callback, sampling_callback, elicitation_callback) at initialization, but provides no public API to update them after the session is created.

This means any client that needs to change callbacks at runtime (e.g., updating roots in response to user action) must mutate private attributes like _list_roots_callback directly — which is fragile and couples consumers to implementation details.

Use case

A client connects to a server with initial roots, then the user changes the working directory or project context. The client needs to update its roots callback so that the next roots/list request from the server reflects the new roots. Today this requires:

session._list_roots_callback = new_callback  # private attribute

The same issue applies to _sampling_callback and _elicitation_callback.

Proposed solution

Add public setter methods on ClientSession for updating callbacks after initialization. For example:

session.set_list_roots_callback(callback)
session.set_sampling_callback(callback)
session.set_elicitation_callback(callback)

Or alternatively, make the callback attributes public (without the leading underscore).

Context

This came up while fixing PrefectHQ/fastmcp#326 — Client.set_roots() wasn't updating the live session because it only modified pending kwargs. The fix (PrefectHQ/fastmcp#3714) mutates _list_roots_callback directly with a comment noting the fragility. A public API would make this safe and stable.