fix(diagnostics): if buffer not loaded, skip handlers that set extmark (#25628)

Problem:
When enabling diagnostics, there can be diagnostics for unloaded buffer,
but some handlers nevertheless attempt to set extmarks in such buffers.

Solution:
* Exit underline/virtual_text handler if buffer is not loaded.
* Don't require is_loaded as precondition for show(), because handlers
  don't necessarily depend on it.

runtime/lua/vim/diagnostic.lua

@@ -652,20 +652,18 @@ function M.config(opts, namespace)
 
   if namespace then
     for bufnr, v in pairs(diagnostic_cache) do
-      if api.nvim_buf_is_loaded(bufnr) and v[namespace] then
+      if v[namespace] then
         M.show(namespace, bufnr)
       end
     end
   else
     for bufnr, v in pairs(diagnostic_cache) do
-      if api.nvim_buf_is_loaded(bufnr) then
       for ns in pairs(v) do
         M.show(ns, bufnr)
       end
     end
   end
 end
-end
 
 --- Set diagnostics for the given namespace and buffer.
 ---
@@ -693,9 +691,7 @@ function M.set(namespace, bufnr, diagnostics, opts)
     set_diagnostic_cache(namespace, bufnr, diagnostics)
   end
 
-  if api.nvim_buf_is_loaded(bufnr) then
   M.show(namespace, bufnr, nil, opts)
-  end
 
   api.nvim_exec_autocmds('DiagnosticChanged', {
     modeline = false,
@@ -928,6 +924,10 @@ M.handlers.underline = {
     bufnr = get_bufnr(bufnr)
     opts = opts or {}
 
+    if not vim.api.nvim_buf_is_loaded(bufnr) then
+      return
+    end
+
     if opts.underline and opts.underline.severity then
       diagnostics = filter_by_severity(opts.underline.severity, diagnostics)
     end
@@ -994,6 +994,10 @@ M.handlers.virtual_text = {
     bufnr = get_bufnr(bufnr)
     opts = opts or {}
 
+    if not vim.api.nvim_buf_is_loaded(bufnr) then
+      return
+    end
+
     local severity
     if opts.virtual_text then
       if opts.virtual_text.format then