Validate protocol_version value

## Motivation and Context

Currently, MCP supports the following three protocol versions:
"2025-06-18", "2025-03-26", and "2024-11-05".
https://modelcontextprotocol.io/introduction

As shown in the changes to `test/mcp/configuration_test.rb`,
the `protocol_version` value is not currently being validated,
which allows unexpected `protocol_version` values to be accepted.

This PR ensures that only supported protocol versions are accepted.

## How Has This Been Tested?

Existing tests have been updated and new tests have been added.

## Breaking Changes

None. If it becomes an issue, it means a value that was not originally intended has been used.

## Additional context

While one possible approach might be to fall back to a default protocol version
when an invalid `protocol_version` value is provided,
this PR opts to reject invalid `protocol_version` values explicitly.
The reasoning is that encouraging users to provide intentional,
valid input is preferable to silently falling back to a default,
which could lead to unexpected behavior.

test/mcp/server_test.rb

@@ -804,7 +804,7 @@ module MCP
     end
 
     test "server protocol version can be overridden via configuration" do
-      custom_version = "2025-03-27"
+      custom_version = "2025-03-26"
       configuration = Configuration.new(protocol_version: custom_version)
       server = Server.new(name: "test_server", configuration: configuration)