test(lsp): fix fake LSP server timeout not working (#37970)

Problem:  Fake LSP server does not timeout or respond to SIGTERM as it
          does not run the event loop.
Solution: Instead of io.read(), use stdioopen()'s on_stdin callback to
          accumulate input and use vim.wait() to wait for input.

Also, in the test suite, don't stop a session when it's not running, as
calling uv.stop() outside uv.run() will instead cause the next uv.run()
to stop immediately, which cancels the next RPC request.

test/functional/plugin/lsp_spec.lua

@@ -88,7 +88,11 @@ describe('LSP', function()
 
   after_each(function()
     stop()
-    exec_lua('vim.iter(lsp.get_clients()):each(function(client) client:stop(true) end)')
+    exec_lua(function()
+      vim.iter(vim.lsp.get_clients({ _uninitialized = true })):each(function(client)
+        client:stop(true)
+      end)
+    end)
     api.nvim_exec_autocmds('VimLeavePre', { modeline = false })
   end)
 
@@ -206,18 +210,18 @@ describe('LSP', function()
     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({
+      local clients = exec_lua(function()
+        local client1 = assert(vim.lsp.start({
           name = 'dup-test',
           cmd = { vim.v.progpath, '-l', fake_lsp_code, 'basic_init' },
-        }, { attach = false })
+        }, { attach = false }))
         vim.lsp.get_client_by_id(client1):stop()
-        local client2 = vim.lsp.start({
+        local client2 = assert(vim.lsp.start({
           name = 'dup-test',
           cmd = { vim.v.progpath, '-l', fake_lsp_code, 'basic_init' },
-        }, { attach = false })
+        }, { attach = false }))
         return { client1, client2 }
-        ]])
+      end)
       local c1, c2 = clients[1], clients[2]
       eq(false, c1 == c2, 'Expected a fresh client while the old one is stopping')
     end)
@@ -320,14 +324,8 @@ describe('LSP', function()
     )
 
     it('should succeed with manual shutdown', function()
-      if is_ci() then
-        pending('hangs the build on CI #14028, re-enable with freeze timeout #14204')
-        return
-      elseif t.skip_fragile(pending) then
-        return
-      end
       local expected_handlers = {
-        { NIL, {}, { method = 'shutdown', bufnr = 1, client_id = 1, version = 0 } },
+        { NIL, {}, { method = 'shutdown', bufnr = 1, client_id = 1, request_id = 2, version = 0 } },
         { NIL, {}, { method = 'test', client_id = 1 } },
       }
       test_rpc_server {