fix: unsupported header takes precedence over header mismatch

[Yuan325]

The sep-2575-http-server-header-mismatch-400 should check with a supported header and mismatch protocol version in _meta.

According to the SEP,

* Requirement: This header is MANDATORY. Servers should reject requests with a missing or unsupported version.
* This header MUST match the value provided in the Request as specified below.

hence, unsupported header takes precedence over header mismatch. In order to run this check, we must provide a supported header version with a mismatch _meta.protocolVersion value.

[pkg-pr-new]

Open in StackBlitz

npx https://pkg.pr.new/@modelcontextprotocol/conformance@293

commit: 77594b4

[Yuan325]

@pcarleton If you can take a look at this that would be great~ Thank you!

[pcarleton]

Lgtm, ty!