bug#81514: 32.0.50; Eglot messages progress reporter shows errors

I checked out the latest patch, but there are still some issues.

According to the LSP specification, `$/progress` notifications of kind
`begin` or `report` may omit the `percentage` field:

 > Optional progress percentage to display (value 100 is considered 100%).
 > If not provided, infinite progress is assumed, and clients are allowed
 > to ignore the `percentage` value in subsequent report notifications.

A notification of kind `end` does not contain a `percentage` field.

rust-analyzer, at least, sends many progress notifications without a
percentage, for both `begin` and `report` events. As a result, both the
current `master` and `emacs-31` branches signal numerous errors.

Perhaps a separate, non-numeric progress reporter could be introduced
for progress without a percentage. Alternatively, if supporting this is
too complicated for older Emacs versions, such notifications could
simply be ignored.