## Motivation and Context
`lib/mcp.rb` currently declares all `require_relative` statements in one place.
As a result, Server and Client are both loaded, the stdio and Streamable HTTP
transports are both loaded, and Tool, Prompt, and Resource are all loaded,
regardless of which components are actually used.
These components are selected by the user depending on their use case,
so loading everything upfront is unnecessary.
Using `autoload` for selected constants defers loading until the constant
is actually referenced, so only the subsystems required by the user are loaded.
## How Has This Been Tested?
All existing tests pass unchanged.
## Breaking Change
None. This change only defers loading of unused components.
Previously, the `icons` parameter was always included in responses (as an empty array)
even when not configured. This differs from the response representation in MCP Ruby SDK 0.4.0.
Since `icons` is optional, it should be omitted from responses when unused to reduce
unnecessary context window usage.
https://modelcontextprotocol.io/specification/2025-11-25/schema#implementation
This fix applies to `Tool#to_h`, `Prompt#to_h`, `Resource#to_h`, `ResourceTemplate#to_h`,
and `Server#server_info`.
It doesn't seem critical, but if there are no other issues, it would be good to cut 0.5.1 bugfix release soon.
