Rename FastMCP to MCPServer and restructure constructor

[maxisbey]

Summary

Rename FastMCP to MCPServer (or similar) and restructure so that transport-specific options are passed to run() rather than the constructor.

Motivation

• The name "FastMCP" was inherited from the original external project but doesn't clearly convey what it is
• The constructor currently accepts many HTTP-specific options (host, port, stateless_http, etc.) that only apply when running with a specific transport
• This couples server definition with transport configuration

Goal

• Clear, descriptive name for the high-level server class
• Constructor focused on server identity and capabilities
• Transport-specific options moved to run() or transport configuration

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