refactor(tests): merge n.spawn/n.spawn_argv into n.new_session #31859

Problem:
- `n.spawn()` is misleading because it also connects RPC, it's not just
  "spawning" a process.
- It's confusing that `n.spawn()` and `n.spawn_argv()` are separate.

Solution:
- Replace `n.spawn()`/`n.spawn_argv()` with a single function `n.new_session()`.
  This name aligns with the existing functions `n.set_session`/`n.get_session`.
    - Note: removes direct handling of `prepend_argv`, but I doubt that was
      important or intentional. If callers want to control use of `prepend_argv`
      then we should add a new flag to `test.session.Opts`.
- Move `keep` to first parameter of `n.new_session()`.
- Add a `merge` flag to `test.session.Opts`
- Mark `_new_argv()` as private. Test should use clear/new_session/spawn_wait
  instead.

test/functional/api/server_requests_spec.lua

@@ -9,7 +9,6 @@ local nvim_prog, command, fn = n.nvim_prog, n.command, n.fn
 local source, next_msg = n.source, n.next_msg
 local ok = t.ok
 local api = n.api
-local spawn, merge_args = n.spawn, n.merge_args
 local set_session = n.set_session
 local pcall_err = t.pcall_err
 local assert_alive = n.assert_alive
@@ -281,10 +280,9 @@ describe('server -> client', function()
   end)
 
   describe('connecting to another (peer) nvim', function()
-    local nvim_argv = merge_args(n.nvim_argv, { '--headless' })
     local function connect_test(server, mode, address)
       local serverpid = fn.getpid()
-      local client = spawn(nvim_argv, false, nil, true)
+      local client = n.new_session(true)
       set_session(client)
 
       local clientpid = fn.getpid()
@@ -312,7 +310,7 @@ describe('server -> client', function()
     end
 
     it('via named pipe', function()
-      local server = spawn(nvim_argv)
+      local server = n.new_session(false)
       set_session(server)
       local address = fn.serverlist()[1]
       local first = string.sub(address, 1, 1)
@@ -321,7 +319,7 @@ describe('server -> client', function()
     end)
 
     it('via ipv4 address', function()
-      local server = spawn(nvim_argv)
+      local server = n.new_session(false)
       set_session(server)
       local status, address = pcall(fn.serverstart, '127.0.0.1:')
       if not status then
@@ -332,7 +330,7 @@ describe('server -> client', function()
     end)
 
     it('via ipv6 address', function()
-      local server = spawn(nvim_argv)
+      local server = n.new_session(false)
       set_session(server)
       local status, address = pcall(fn.serverstart, '::1:')
       if not status then
@@ -343,7 +341,7 @@ describe('server -> client', function()
     end)
 
     it('via hostname', function()
-      local server = spawn(nvim_argv)
+      local server = n.new_session(false)
       set_session(server)
       local address = fn.serverstart('localhost:')
       eq('localhost:', string.sub(address, 1, 10))
@@ -351,10 +349,10 @@ describe('server -> client', function()
     end)
 
     it('does not crash on receiving UI events', function()
-      local server = spawn(nvim_argv)
+      local server = n.new_session(false)
       set_session(server)
       local address = fn.serverlist()[1]
-      local client = spawn(nvim_argv, false, nil, true)
+      local client = n.new_session(true)
       set_session(client)
 
       local id = fn.sockconnect('pipe', address, { rpc = true })