fix(lsp): check if client is stopping before reuse #33796

Problem:
Stopping a language server and then calling vim.lsp.start() with the same name/root will return the old language server that's in the middle of shutting down. vim.lsp.start() won't return a new server until the old process has terminated.

Solution:
Introducing a client._is_stopping field that tracks the shutdown phase, preventing the client from being reused.

test/functional/plugin/lsp_spec.lua

@@ -218,6 +218,25 @@ describe('LSP', function()
         )
       end)
     end)
+
+    it('does not reuse an already-stopping client #33616', function()
+      -- we immediately try to start a second client with the same name/root
+      -- before the first one has finished shutting down; we must get a new id.
+      local clients = exec_lua([[
+        local client1 = vim.lsp.start({
+          name = 'dup-test',
+          cmd = { vim.v.progpath, '-l', fake_lsp_code, 'basic_init' },
+        }, { attach = false })
+        vim.lsp.get_client_by_id(client1):stop()
+        local client2 = vim.lsp.start({
+          name = 'dup-test',
+          cmd = { vim.v.progpath, '-l', fake_lsp_code, 'basic_init' },
+        }, { attach = false })
+        return { client1, client2 }
+        ]])
+      local c1, c2 = clients[1], clients[2]
+      eq(false, c1 == c2, 'Expected a fresh client while the old one is stopping')
+    end)
   end)
 
   describe('basic_init test', function()