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

src/mcp/client/client.py の raise_exceptions TODO から始め、インメモリクライアントと HTTP クライアントの現在の挙動を、async with Client(...) における ExceptionGroup の処理も含めて追跡します。これを docs/troubleshooting.md のガイダンスと比較し、確認できたセマンティクスを docs/client/ および/または docs/troubleshooting.md にドキュメント化します。ユーザーが影響を受ける失敗を特定し、推奨されるパターンとコンテキストマネージャー内で MCPError を捕捉する方法のどちらかを選択できれば完了です。