docs: clarify when Client(raise_exceptions=True) actually raises

Description

Client exposes raise_exceptions: bool = False with a code TODO asking when raise_exceptions=True actually raises:

https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/client/client.py (around the raise_exceptions field)

What's missing

There is no clear user-facing documentation of:

1. Which failure modes are affected (in-process server exceptions vs transport/protocol errors)
2. How this interacts with ExceptionGroup wrapping from async with Client(...)
3. Recommended patterns vs catching MCPError inside the context manager (as docs/troubleshooting.md already teaches)

Proposed work

• Trace current behavior on main for in-memory and HTTP clients
• Document the real semantics in docs/client/ and/or docs/troubleshooting.md
• Add a small docs_src example and/or regression test if behavior is surprising
• Only change runtime behavior if docs reveal a clear bug; prefer docs-first

Related

• TODO(Marcelo) on Client.raise_exceptions in src/mcp/client/client.py
• #1401 ClientSession Error Handling (different surface; has an open PR)
• troubleshooting ExceptionGroup guidance

从 src/mcp/client/client.py 中的 raise_exceptions TODO 开始，跟踪内存中客户端和 HTTP 客户端的当前行为，包括 async with Client(...) 中对 ExceptionGroup 的处理。将其与 docs/troubleshooting.md 中的指导进行比较，然后在 docs/client/ 和/或 docs/troubleshooting.md 中记录已确认的语义。完成标准是用户能够识别受影响的失败，并在推荐模式与在 context manager 内捕获 MCPError 之间进行选择。