fix(treesitter): conceal_lines is not applied to injected trees

Problem:
TSHighlighter._on_conceal_line() parses with the range { row, row }. A
Range2 has an exclusive end, so that is the empty range, and no injected
region ever intercepts it.

The root tree is parsed regardless, because its region is empty, so only
injections are affected: conceal_lines metadata coming from an injected
language's highlights query is dropped. on_range_impl() then records the
row in _conceal_checked, so the miss persists until the buffer changes.

For a markdown code block nested in a markdown code block, the inner
fence delimiters stay visible and nvim_win_text_height() reports 5 rows
where 3 are displayed.

Solution:
Pass the one-row range, as the on_range_impl() call below already does.

AI-assisted

runtime/lua/vim/treesitter/highlighter.lua

@@ -549,7 +549,7 @@ function TSHighlighter._on_conceal_line(_, _, buf, row)
 
   -- Do not affect potentially populated highlight state.
   local highlight_states = self._highlight_states
-  self.tree:parse({ row, row })
+  self.tree:parse({ row, row + 1 })
   self:prepare_highlight_states(row, row)
   on_range_impl(self, buf, row, 0, row + 1, 0, false, true)
   self._highlight_states = highlight_states

test/functional/treesitter/highlight_spec.lua

@@ -1553,3 +1553,30 @@ mispelledtwo]])
   local pos = api.nvim_win_get_cursor(0)
   eq(1, pos[1], 'Should have wrapped back to Line 1')
 end)
+
+it('conceals lines contributed by an injected tree', function()
+  clear()
+  command('set conceallevel=3')
+
+  -- Measured without an intervening redraw, so that the conceal_line callback
+  -- is the thing that has to parse the injection. The outer ~~~ fences come
+  -- from the root tree, the inner ``` fences from the markdown tree injected
+  -- into it; all four carry conceal_lines, leaving "filler", "print(1)" and
+  -- "tail" on screen.
+  eq(
+    3,
+    exec_lua(function()
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, {
+        'filler',
+        '~~~markdown',
+        '```lua',
+        'print(1)',
+        '```',
+        '~~~',
+        'tail',
+      })
+      vim.treesitter.start(0, 'markdown')
+      return vim.api.nvim_win_text_height(0, {}).all
+    end)
+  )
+end)