fix: lsp.enable() don't work correctly inside FileType event #37538

Problem:
Two cases lsp.enable() won't work in the first FileType event
1. lsp.enable luals inside FileType or ftplugin/lua.lua, then:
```
nvim a.lua
```

2. lsp.enable luals inside FileType or ftplugin/lua.lua, then:
```
nvim -> :edit a.lua -> :mksession! | restart +qa! so Session.vim
```

Solution:
Currently `v:vim_did_enter` is used to detected two cases:
1. "maunally enabled" (lsp.enable() or `:lsp enable`)
2. "inside FileType event"

To detect 2. correctly we use did_filetype().

runtime/lua/vim/lsp.lua

@@ -590,7 +590,7 @@ function lsp.enable(name, enable)
 
   -- Ensure any pre-existing buffers start/stop their LSP clients.
   if enable ~= false then
-    if vim.v.vim_did_enter == 1 and next(lsp._enabled_configs) then
+    if (vim.v.vim_did_enter == 1 or vim.fn.did_filetype() == 1) and next(lsp._enabled_configs) then
       vim.cmd.doautoall('nvim.lsp.enable FileType')
     end
   else

test/functional/plugin/lsp_spec.lua

@@ -6874,6 +6874,40 @@ describe('LSP', function()
       )
     end)
 
+    it('in first FileType event (on startup)', function()
+      local tmp = tmpname()
+      write_file(tmp, string.dump(create_server_definition))
+      n.clear({
+        args = {
+          '--cmd',
+          string.format([[lua assert(loadfile(%q))()]], tmp),
+          '--cmd',
+          [[lua _G.server = _G._create_server({ handlers = {initialize = function(_, _, callback) callback(nil, {capabilities = {}}) end} })]],
+          '--cmd',
+          [[lua vim.lsp.config('foo', { cmd = _G.server.cmd, filetypes = { 'foo' }, root_markers = { '.foorc' } })]],
+          '--cmd',
+          [[au FileType * ++once lua vim.lsp.enable('foo')]],
+          '-c',
+          'set ft=foo',
+        },
+      })
+
+      eq(
+        { 1, 'foo' },
+        exec_lua(function()
+          local foos = vim.lsp.get_clients({ bufnr = 0 })
+          return { #foos, (foos[1] or {}).name }
+        end)
+      )
+      exec_lua([[vim.lsp.enable('foo', false)]])
+      eq(
+        0,
+        exec_lua(function()
+          return #vim.lsp.get_clients({ bufnr = 0 })
+        end)
+      )
+    end)
+
     it('does not attach to buffers more than once if no root_dir', function()
       exec_lua(create_server_definition)