feat(lsp): detach LSP clients when 'filetype' changes #33707

Problem:
When the buffer 'filetype' changes, invalid or non-applicable LSP
clients are not detached.

https://github.com/neovim/neovim/issues/33443
https://github.com/neovim/nvim-lspconfig/issues/3326

Solution:
In the enable() routine, check can_start() on _existing_ clients.

test/functional/plugin/lsp_spec.lua

@@ -6504,6 +6504,58 @@ describe('LSP', function()
       end)
     end)
 
+    it('starts correct LSP and stops incorrect LSP when filetype changes', function()
+      exec_lua(create_server_definition)
+
+      local tmp1 = t.tmpname(true)
+
+      exec_lua(function()
+        local server = _G._create_server({
+          handlers = {
+            initialize = function(_, _, callback)
+              callback(nil, { capabilities = {} })
+            end,
+          },
+        })
+
+        vim.lsp.config('foo', {
+          cmd = server.cmd,
+          filetypes = { 'foo' },
+          root_markers = { '.foorc' },
+        })
+
+        vim.lsp.config('bar', {
+          cmd = server.cmd,
+          filetypes = { 'bar' },
+          root_markers = { '.foorc' },
+        })
+
+        vim.lsp.enable('foo')
+        vim.lsp.enable('bar')
+
+        vim.cmd.edit(tmp1)
+      end)
+
+      local count_clients = function()
+        return exec_lua(function()
+          local foos = vim.lsp.get_clients({ name = 'foo', bufnr = 0 })
+          local bars = vim.lsp.get_clients({ name = 'bar', bufnr = 0 })
+          return { #foos, 'foo', #bars, 'bar' }
+        end)
+      end
+
+      -- No filetype on the buffer yet, so no LSPs.
+      eq({ 0, 'foo', 0, 'bar' }, count_clients())
+
+      -- Set the filetype to 'foo', confirm a LSP starts.
+      exec_lua([[vim.bo.filetype = 'foo']])
+      eq({ 1, 'foo', 0, 'bar' }, count_clients())
+
+      -- Set the filetype to 'bar', confirm a new LSP starts, and the old one goes away.
+      exec_lua([[vim.bo.filetype = 'bar']])
+      eq({ 0, 'foo', 1, 'bar' }, count_clients())
+    end)
+
     it('validates config on attach', function()
       local tmp1 = t.tmpname(true)
       exec_lua(function()