Audit MCPServer constructor parameters

From the V2 checklist in #1253:

We need to recheck the constructor parameters of the MCPServer class. I did remove and refactor a bit, but we need to audit it a bit better.

PR #1732 renamed FastMCP to MCPServer and moved transport-specific options (host, port, stateless_http, etc.) to run(). However, the constructor still has parameters that may need further review:

• Are all remaining constructor parameters appropriate for the constructor vs. run()?
• Are there parameters that should be removed, renamed, or have their defaults reconsidered?
• Is the constructor signature clean and minimal for the common case?

This is a follow-up audit to ensure the MCPServer API surface is well-designed before V2 stabilizes.

Ref: Kludex's V2 checklist in #1253, follow-up to #1732

まず MCPServer のコンストラクターとその run() エントリーポイントを確認し、PR #1732 の変更および #1253 の V2 チェックリストも確認します。各 API にどのパラメーターを含めるべきかを監査し、その結果得られた判断を文書化またはテストして、コンストラクターを最小限かつ V2 に適したものにします。