fix(lsp): send didClose, didOpen when languageId changes #39499

Problem:
If a buffer's filetype changes after the LSP client has already
attached (e.g. from json to jsonc via a modeline), but the client
supports both filetypes, it stays attached. It does not notify the
server of the new languageId, causing the server to incorrectly process
the file using the old languageId.

Solution:
Save the languageId used during textDocument/didOpen, and send
textDocument/didClose + textDocument/didOpen when buffer's languageId
changed.

Lsp spec:
https://github.com/microsoft/language-server-protocol/blob/0003fb53f18dc7a4ac7e51d0eb518deeea90fde5/_specifications/lsp/3.18/textDocument/didOpen.md#L5
> If the language id of a document changes, the client
> needs to send a textDocument/didClose to the server followed by a
> textDocument/didOpen with the new language id if the server handles
> the new language id as well.

AI-assisted: Gemini 3.1 Pro

test/functional/plugin/lsp_spec.lua

@@ -4488,6 +4488,52 @@ describe('LSP', function()
       eq({ 0, 'foo', 1, 'bar' }, count_clients())
     end)
 
+    it('sends didClose and didOpen when languageId changes', function()
+      exec_lua(create_server_definition)
+
+      local tmp1 = t.tmpname(true)
+
+      exec_lua(function()
+        _G.server = _G._create_server({
+          handlers = {
+            initialize = function(_, _, callback)
+              callback(nil, { capabilities = { textDocumentSync = { openClose = true } } })
+            end,
+          },
+        })
+        vim.lsp.config('foo', { cmd = _G.server.cmd, filetypes = { 'foo', 'bar' } })
+        vim.lsp.enable('foo')
+        vim.cmd.edit(tmp1)
+      end)
+
+      local function test_messages()
+        local opens = 0
+        local closes = 0
+        local msgs = exec_lua([[ return _G.server.messages ]])
+        local num_clients = exec_lua([[ return #vim.lsp.get_clients() ]])
+        for _, msg in ipairs(msgs) do
+          opens = opens + (msg.method == 'textDocument/didOpen' and 1 or 0)
+          closes = closes + (msg.method == 'textDocument/didClose' and 1 or 0)
+        end
+        return { opens, 'did_open', closes, 'did_close', num_clients, 'clients' }
+      end
+
+      -- No filetype on the buffer yet.
+      eq({ 0, 'did_open', 0, 'did_close', 0, 'clients' }, test_messages())
+
+      -- Set the filetype to 'foo', confirm didOpen is sent.
+      exec_lua([[vim.bo.filetype = 'foo']])
+      retry(nil, 1000, function()
+        eq({ 1, 'did_open', 0, 'did_close', 1, 'clients' }, test_messages())
+      end)
+
+      -- Set to anohter lsp-supported filetype 'bar', confirm didClose and didOpen are sent.
+      exec_lua([[vim.bo.filetype = 'bar']])
+      retry(nil, 1000, function()
+        eq({ 2, 'did_open', 1, 'did_close', 1, 'clients' }, test_messages())
+      end)
+    end)
+
     it('validates config on attach', function()
       local tmp1 = t.tmpname(true)
       exec_lua(function()