fix(lsp): don't overlay insertion-style inline completions (#36477)

* feat(lua): `Range:is_empty()` to check vim.range emptiness

* fix(lsp): don't overlay insertion-style inline completions

**Problem:** Some servers commonly respond with an empty inline
completion range which acts as a position where text should be inserted.
However, the inline completion module assumes that all responses with a
range are deletions + insertions that thus require an `overlay` display
style. This causes an incorrect preview, because the virtual text should
have the `inline` display style (to reflect that this is purely an
insertion).

**Solution:** Only use `overlay` for non-empty replacement ranges.

test/functional/plugin/lsp/inline_completion_spec.lua

@@ -83,6 +83,27 @@ describe('vim.lsp.inline_completion', function()
         },
         handlers = {
           ['textDocument/inlineCompletion'] = function(_, _, callback)
+            if _G.empty then
+              callback(nil, {
+                items = {
+                  {
+                    insertText = 'foobar',
+                    range = {
+                      start = {
+                        line = 0,
+                        character = 19,
+                      },
+                      ['end'] = {
+                        line = 0,
+                        character = 19,
+                      },
+                    },
+                  },
+                },
+              })
+              return
+            end
+
             callback(nil, {
               items = {
                 {
@@ -198,6 +219,19 @@ describe('vim.lsp.inline_completion', function()
       screen:expect({ grid = grid_applied_candidates })
     end)
 
+    it('correctly displays with absent/empty range', function()
+      exec_lua(function()
+        _G.empty = true
+      end)
+      feed('I')
+      screen:expect([[
+        function fibonacci({1:foobar})                           |
+        ^                                                     |
+        {1:~                                                    }|*11
+        {3:-- INSERT --}                                         |
+      ]])
+    end)
+
     it('accepts on_accept callback', function()
       feed('i')
       screen:expect({ grid = grid_with_candidates })