bug#61764: 29.0.60; eglot show incomplete inlay hints

[Kai Ma <justksqsf@gmail.com>]

The display of some inlay hints are not complete.  For example, in a
Rust source file:

  let v = vec![1i32];

The expected display is:

  let v: Vec<i32> = ...;

However, the current eglot (commit 55d29c9bacb in branch emacs-29) only
shows the semicolon:

  let v: = ...;

I found that the inlay hint returned by rust-analyzer is something like:

  ([(:value ": ") (:value "Vec" :location ...) (:value "<i32>")])

And I can confirm that by making the following change, the inlay hints
from rust-analyzer will be displayed correctly.

--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -3549,7 +3549,7 @@ eglot--update-hints-1
                   (left-pad (and paddingLeft (not (memq (char-before) '(32 9)))))
                   (right-pad (and paddingRight (not (memq (char-after) '(32 9)))))
                   (text (if (stringp label)
-                            label (plist-get (elt label 0) :value))))
+                            label (mapconcat (lambda (plist) (plist-get plist :value)) label))))
               (overlay-put ov 'before-string
                            (propertize
                             (concat (and left-pad " ") text (and right-pad " "))

According to the LSP spec [1], the intention of this design is to
support the use case where a LSP client shows different information for
each part.  Therefore, I believe this (concatenation) is the correct
thing to do, regardless of whether eglot supports hovering.

[1] https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#inlayHint

Kai