bug#70724: 29.2.50; eglot-reconnect errors when the project is deleted

[Spencer Baugh <sbaugh@janestreet.com>]

In some project /home/foo/proj, with pretty much any LSP server:

1. In /home/foo/proj, M-x eglot, starting some LSP server

2. Delete the directory /home/foo/proj

3. The LSP server will crash/exit

4. The process sentinel for the server will run, running
   eglot--on-shutdown which by default will call eglot-reconnect

5. eglot-reconnect extracts the saved project instance out of the
   server, which has a root directory which no longer exists, and calls
   eglot--connect with it

6. eglot--connect calls project-name on a nonexistent project instance,
   which may fail with an error depending on the project implementation
   (I have a custom project implementation, but I think this can happen
   with project-vc too)

7. This causes the process sentinel to error.

I think the right fix is probably for eglot--on-shutdown (or maybe
eglot-reconnect) to call (project-current nil (project-root pr)) to find
the new project instance.  If that returns nil, the project has
disappeared, and eglot should just not try to reconnect.  This also
would make eglot behave better if the project layout changes (e.g. if
there are nested projects).

Alternatively, maybe eglot--on-shutdown shouldn't automatically
reconnect in the first place?  Maybe reconnection should happen
automatically only when some specific buffer tries to interact with the
LSP - then it can run project-current in the context of that specific
buffer, and see there's no project, and fail.  Plus, if the user kills
all the buffers in the project (possibly with project-kill-buffers)
before deleting it, this approach would entirely avoid the unnecessary
eglot reconnection attempt.


In GNU Emacs 29.2.50 (build 2, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.15.12, Xaw scroll bars) of 2024-04-25 built on
 igm-qws-u22796a
Repository revision: d07451c1f8053fa355d091351a614f232995ab8c
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Rocky Linux 8.9 (Green Obsidian)

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