Send an MCP-Protocol-Version header when using the client Streamable HTTP transport

[halter73]

modelcontextprotocol/modelcontextprotocol#548 updated the MCP specification to require the following:

If using HTTP, the client MUST include the MCP-Protocol-Version: <protocol-version> HTTP header during any subsequent requests to the MCP
server, allowing the MCP server to respond based on the MCP protocol version.

The protocol version sent by the client SHOULD be the one negotiated during initialization.

If the server receives a request with a missing, invalid, or unsupported
MCP-Protocol-VERSION, it MUST respond with 400 Bad Request.
For example: MCP-Protocol-Version: 2024-11-05

We should update the client Streamable HTTP transport to send this header.