`ClientSessionGroup` has no routed `read_resource` / `get_prompt` (only `call_tool`)

Look at the ClientSessionGroup class to understand how call_tool works and the existing reverse indexes for tools. Identify where resources and prompts are aggregated. The new methods read_resource and get_prompt should follow the same routing pattern, using the component_name_hook. Check the disconnect_from_server method to ensure cleanup. Start by examining the session aggregation logic and the existing indexes.

Description

### What I ran into

`ClientSessionGroup` aggregates tools, resources, and prompts into one view, and
`call_tool(name, ...)` routes to the session that owns the tool. But that routing
exists only for tools. There's no equivalent way to read an aggregated resource
or fetch an aggregated prompt through the group.

### Why it matters

To read a resource or get a prompt, a caller has to track the owning
`ClientSession` themselves and bypass the group, which defeats the "one object,
one view" purpose of the abstraction. The asymmetry is surprising given
`call_tool` is already there.

### Proposed change

Add `read_resource(name, ...)` and `get_prompt(name, arguments, ...)` mirroring
`call_tool`: resolve the aggregate key (honoring `component_name_hook`) to the
owning session via resource/prompt reverse indexes, forward the real wire
URI/name, carry the same `allow_input_required` overloads, and clean the indexes
up on `disconnect_from_server`.

### Example

```python
result = await group.read_resource("Library.hours")
prompt = await group.get_prompt("Greeter.greet", {"name": "Ada"})