[gwyszynski-sa]

The legacy initialize handshake pins params.protocolVersion but sends no
Mcp-Protocol-Version header of its own: setMCPHeaders finds c.initializedResult
still nil, since it is assigned a few lines later, and the message carries no
_meta.protocolVersion, so the header falls to the request context. For a
standalone client that context is empty and the header is absent; for a process
that is both a server and a client it holds that process's inbound version,
which contradicts the body it accompanies. This is the client-side mirror of
#963.

No version is negotiated at that point, so the request has none of its own to
state, and the caller's must not stand in for one. Clears it for the initialize
send rather than sending the proposed version: a header the peer does not
support fails the handshake outright, where the body would have negotiated down.
Standalone clients are unaffected, the header having already been absent there.

The fake server asserted wantProtocolVersion for other methods but for none of
its sixteen initialize cases. Asserts centrally that initialize carries no
protocol version header, and adds a Connect whose context already holds one,
without which that assertion cannot fail. Requests after negotiation are left
unchecked there, since which source wins once initializedResult is set is #1162.

Fixes #1164