### 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"})
Description