[mlir-lsp] Log invalid notification params (#89856)

When the `lsp::MessageHandler` processes a request with invalid params
(that is, the "params" JSON sent along with the request does not match
the shape expected by the message handler for the given method), it
replies by sending an error response to the client.

On the other hand, the language server protocol specifies that
notifications must not result in responses. As a result, when the
JSON params accompanying a notification cannot be parsed, no error is
sent back; there is no indication that an error has occurred at all.

This patch adds an error log for that case. Although clients cannot
parse error logs, this at least provides an indication that something
went wrong on the language server side.