mcp: implement support for JSON responses in the MCP streamable server

Add a new (currently unexported) jsonResponse option to
StreamableServerTransportOptions, and use it to control the response
content type, serving application/json if set.

Additionally:
- A transportOptions field is added to the StreamableHTTPOptions.
- A messages iterator is added to encapsulate the iteration of stream
  messages, since the handling of JSON and SSE responses are otherwise
  very different.
- The serving flow is refactored to avoid returning (statusCode,
  message), primarily because this seemed liable to lead to redundant
  calls to WriteHeader, because only local logic knows whether or not
  any data has been written to the response.
- The serving flow is refactored to delegate to responseJSON and
  responseSSE, according to the currently unexported jsonResponse
  option.
- A bug is fixed where all GET streams were considered persistent: the
  terminal condition req.Method == http.MethodPost && nOutstanding == 0
  was not right: GET requests may implement stream resumption.

Updates #211

mcp/shared.go

@@ -275,6 +275,8 @@ func sessionMethod[S Session, P Params, R Result](f func(S, context.Context, P)
 
 // Error codes
 const (
+	// TODO: should these be unexported?
+
 	CodeResourceNotFound = -32002
 	// The error code if the method exists and was called properly, but the peer does not support it.
 	CodeUnsupportedMethod = -31001