Describe the bug
In #548, language was added to make MCP-Protocol-Version: required. #668 added language that if the header is not sent, the server SHOULD assume version 2025-03-26.

However the current version of the spec 2025-06-18 reads:

For backwards compatibility, if the server does not receive an MCP-Protocol-Version header, and has no other way to identify the version - for example, by relying on the protocol version negotiated during initialization - the server SHOULD assume protocol version 2025-06-18.

However this logically doesn't make sense -- if the client is running 2025-06-18, it MUST include the header. If the header is missing, the correct assumption should be 2025-03-26 (as the only version where MCP-Protocol-Version is not required).

Additionally, I think the language around this should be lessened to "MAY" -- or perhaps this line can be removed altogether, so that servers can enforce the requirement if they support versions +2025-06-18.

Expected behavior
Update the line to be:

For backwards compatibility, if the server does not receive an MCP-Protocol-Version header, and has no other way to identify the version - for example, by relying on the protocol version negotiated during initialization - the server MAY assume protocol version 2025-03-26.

Logs
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Additional context
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