test: spawn_wait() starts a non-RPC Nvim process

Problem:
Can't use `n.clear()` to test non-RPC `nvim` invocations. So tests end
up creating ad-hoc wrappers around `system()` or `jobstart()`.

Solution:
- Introduce `n.spawn_wait()`
- TODO (followup PR): Rename `n.spawn()` and `n.spawn_wait()`.
  It's misleading that `n.spawn()` returns a RPC session...

test/client/session.lua

@@ -1,13 +1,21 @@
-local uv = vim.uv
-local MsgpackRpcStream = require('test.client.msgpack_rpc_stream')
+---
+--- Nvim msgpack-RPC protocol session. Manages requests/notifications/responses.
+---
 
+local uv = vim.uv
+local RpcStream = require('test.client.rpc_stream')
+
+--- Nvim msgpack-RPC protocol session. Manages requests/notifications/responses.
+---
 --- @class test.Session
---- @field private _pending_messages string[]
---- @field private _msgpack_rpc_stream test.MsgpackRpcStream
+--- @field private _pending_messages string[] Requests/notifications received from the remote end.
+--- @field private _rpc_stream test.RpcStream
 --- @field private _prepare uv.uv_prepare_t
 --- @field private _timer uv.uv_timer_t
---- @field private _is_running boolean
 --- @field exec_lua_setup boolean
+--- @field private _is_running boolean true during `Session:run()` scope.
+--- @field private _stdout_buffer string[] Stores stdout chunks
+--- @field public stdout string Full stdout after the process exits
 local Session = {}
 Session.__index = Session
 if package.loaded['jit'] then
@@ -51,9 +59,10 @@ local function coroutine_exec(func, ...)
   end))
 end
 
+--- Creates a new msgpack-RPC session.
 function Session.new(stream)
   return setmetatable({
-    _msgpack_rpc_stream = MsgpackRpcStream.new(stream),
+    _rpc_stream = RpcStream.new(stream),
     _pending_messages = {},
     _prepare = uv.new_prepare(),
     _timer = uv.new_timer(),
@@ -91,10 +100,13 @@ function Session:next_message(timeout)
   return table.remove(self._pending_messages, 1)
 end
 
+--- Sends a notification to the RPC endpoint.
 function Session:notify(method, ...)
-  self._msgpack_rpc_stream:write(method, { ... })
+  self._rpc_stream:write(method, { ... })
 end
 
+--- Sends a request to the RPC endpoint.
+---
 --- @param method string
 --- @param ... any
 --- @return boolean, table
@@ -114,8 +126,16 @@ function Session:request(method, ...)
   return true, result
 end
 
---- Runs the event loop.
+--- Processes incoming RPC requests/notifications until exhausted.
+---
+--- TODO(justinmk): luaclient2 avoids this via uvutil.cb_wait() + uvutil.add_idle_call()?
+---
+--- @param request_cb function Handles requests from the sever to the local end.
+--- @param notification_cb function Handles notifications from the sever to the local end.
+--- @param setup_cb function
+--- @param timeout number
 function Session:run(request_cb, notification_cb, setup_cb, timeout)
+  --- Handles an incoming request.
   local function on_request(method, args, response)
     coroutine_exec(request_cb, method, args, function(status, result, flag)
       if status then
@@ -126,6 +146,7 @@ function Session:run(request_cb, notification_cb, setup_cb, timeout)
     end)
   end
 
+  --- Handles an incoming notification.
   local function on_notification(method, args)
     coroutine_exec(notification_cb, method, args)
   end
@@ -160,39 +181,45 @@ function Session:close(signal)
   if not self._prepare:is_closing() then
     self._prepare:close()
   end
-  self._msgpack_rpc_stream:close(signal)
+  self._rpc_stream:close(signal)
   self.closed = true
 end
 
+--- Sends a request to the RPC endpoint, without blocking (schedules a coroutine).
 function Session:_yielding_request(method, args)
   return coroutine.yield(function(co)
-    self._msgpack_rpc_stream:write(method, args, function(err, result)
+    self._rpc_stream:write(method, args, function(err, result)
       resume(co, err, result)
     end)
   end)
 end
 
+--- Sends a request to the RPC endpoint, and blocks (polls event loop) until a response is received.
 function Session:_blocking_request(method, args)
   local err, result
 
+  -- Invoked when a request is received from the remote end.
   local function on_request(method_, args_, response)
     table.insert(self._pending_messages, { 'request', method_, args_, response })
   end
 
+  -- Invoked when a notification is received from the remote end.
   local function on_notification(method_, args_)
     table.insert(self._pending_messages, { 'notification', method_, args_ })
   end
 
-  self._msgpack_rpc_stream:write(method, args, function(e, r)
+  self._rpc_stream:write(method, args, function(e, r)
     err = e
     result = r
     uv.stop()
   end)
 
+  -- Poll for incoming requests/notifications received from the remote end.
   self:_run(on_request, on_notification)
   return (err or self.eof_err), result
 end
 
+--- Polls for incoming requests/notifications received from the remote end.
 function Session:_run(request_cb, notification_cb, timeout)
   if type(timeout) == 'number' then
     self._prepare:start(function()
@@ -202,14 +229,15 @@ function Session:_run(request_cb, notification_cb, timeout)
       self._prepare:stop()
     end)
   end
-  self._msgpack_rpc_stream:read_start(request_cb, notification_cb, function()
+  self._rpc_stream:read_start(request_cb, notification_cb, function()
     uv.stop()
     self.eof_err = { 1, 'EOF was received from Nvim. Likely the Nvim process crashed.' }
   end)
   uv.run()
   self._prepare:stop()
   self._timer:stop()
-  self._msgpack_rpc_stream:read_stop()
+  self._rpc_stream:read_stop()
 end
 
+--- Nvim msgpack-RPC session.
 return Session