refactor(lsp): encapsulate rpc uv handle

To prepare for different transports like TCP where the handle won't have
a kill method.

runtime/doc/lsp.txt

@@ -1904,7 +1904,7 @@ notify({method}, {params})                              *vim.lsp.rpc.notify()*
 
     Parameters: ~
         {method}  (string) The invoked LSP method
-        {params}  (table): Parameters for the invoked LSP method
+        {params}  (table|nil): Parameters for the invoked LSP method
 
     Return: ~
         (bool) `true` if notification could be sent, `false` if not
@@ -1915,7 +1915,8 @@ request({method}, {params}, {callback}, {notify_reply_callback})
 
     Parameters: ~
         {method}                 (string) The invoked LSP method
-        {params}                 (table) Parameters for the invoked LSP method
+        {params}                 (table|nil) Parameters for the invoked LSP
+                                 method
         {callback}               (function) Callback to invoke
         {notify_reply_callback}  (function|nil) Callback to invoke as soon as
                                  a request is no longer pending

runtime/lua/vim/lsp.lua

@@ -1464,14 +1464,13 @@ function lsp.start_client(config)
   --- you request to stop a client which has previously been requested to
   --- shutdown, it will automatically escalate and force shutdown.
   ---
-  ---@param force (bool, optional)
+  ---@param force boolean|nil
   function client.stop(force)
-    local handle = rpc.handle
-    if handle:is_closing() then
+    if rpc.is_closing() then
       return
     end
     if force or not client.initialized or graceful_shutdown_failed then
-      handle:kill(15)
+      rpc.terminate()
       return
     end
     -- Sending a signal after a process has exited is acceptable.
@@ -1480,7 +1479,7 @@ function lsp.start_client(config)
         rpc.notify('exit')
       else
         -- If there was an error in the shutdown request, then term to be safe.
-        handle:kill(15)
+        rpc.terminate()
         graceful_shutdown_failed = true
       end
     end)
@@ -1492,7 +1491,7 @@ function lsp.start_client(config)
   ---@returns (bool) true if client is stopped or in the process of being
   ---stopped; false otherwise
   function client.is_stopped()
-    return rpc.handle:is_closing()
+    return rpc.is_closing()
   end
 
   ---@private

runtime/lua/vim/lsp/rpc.lua

@@ -405,7 +405,7 @@ local function start(cmd, cmd_args, dispatchers, extra_spawn_params)
   --
   --- Sends a notification to the LSP server.
   ---@param method (string) The invoked LSP method
-  ---@param params (table): Parameters for the invoked LSP method
+  ---@param params (table|nil): Parameters for the invoked LSP method
   ---@returns (bool) `true` if notification could be sent, `false` if not
   local function notify(method, params)
     return encode_and_send({
@@ -432,7 +432,7 @@ local function start(cmd, cmd_args, dispatchers, extra_spawn_params)
   --- Sends a request to the LSP server and runs {callback} upon response.
   ---
   ---@param method (string) The invoked LSP method
-  ---@param params (table) Parameters for the invoked LSP method
+  ---@param params (table|nil) Parameters for the invoked LSP method
   ---@param callback (function) Callback to invoke
   ---@param notify_reply_callback (function|nil) Callback to invoke as soon as a request is no longer pending
   ---@returns (bool, number) `(true, message_id)` if request could be sent, `false` if not
@@ -626,8 +626,12 @@ local function start(cmd, cmd_args, dispatchers, extra_spawn_params)
   end))
 
   return {
-    pid = pid,
-    handle = handle,
+    is_closing = function()
+      return handle:is_closing()
+    end,
+    terminate = function()
+      handle:kill(15)
+    end,
     request = request,
     notify = notify,
   }