It appears that eglot--format-markup only supports MarkedString for
string literals and MarkupContent.  This causes markup provided by the
server as a MarkedString code-block to be formatted using the major
mode.  This happens because the MarkedString code-block uses the
"language" field to designate the type of markup.
eglot--format-markup is only looking for the "kind" field found in
MarkupContent.  The following is an example of a hover response from
an LSP server using a MarkedString code-block to provide a description
in "plaintext".

[jsonrpc] e[13:34:28.888] --> textDocument/hover[25]
{"jsonrpc":"2.0","id":25,"method":"textDocument/hover","params":{"textDocument":{"uri":"file:///home/troy/repos/crates/gtkada_24.0.0_80c56171/src/gtkada.gpr"},"position":{"line":70,"character":12}}}
[jsonrpc] e[13:34:29.033] <-- textDocument/hover[25]
{"jsonrpc":"2.0","id":25,"result":{"contents":[{"language":"plaintext","value":"This
package specifies the compilation options used when building an
executable or a library for a project. Most of the options should be
set in one of Compiler, Binder or Linker packages, but there are some
general options that should be defined in this package."}]}}

The following patch will format MarkedString code-blocks for markdown
and plaintext while using the major mode to format any other language.