fix(health): emit Progress message #37123

Problem:
The `"Running healthchecks..."` message doesn't inform the user much and
is a hack from before we got a way to emit actual progress messages.

Solution:
Use `nvim_echo` to emit progress messages showing the name of the report
that is currently running.

runtime/lua/vim/health.lua

@@ -371,6 +371,24 @@ local function get_summary()
   return s
 end
 
+---Emit progress messages
+---@param len integer
+---@return fun(status: 'success'|'running', idx: integer, fmt: string, ...: any): nil
+local function progress_report(len)
+  local progress = { kind = 'progress', title = 'checkhealth' }
+
+  return function(status, idx, fmt, ...)
+    progress.status = status
+    progress.percent = status == 'success' and nil or math.floor(idx / len * 100)
+    -- percent=0 omits the reporting of percentage, so use 1% instead
+    -- progress.percent = progress.percent == 0 and 1 or progress.percent
+    progress.id = vim.api.nvim_echo({ { fmt:format(...) } }, false, progress)
+    -- extui/ui2 shows all messages at once after the healthchecks are finished.
+    -- This 1ms wait ensures the messages are shown separately
+    vim.wait(1)
+  end
+end
+
 --- Runs the specified healthchecks.
 --- Runs all discovered healthchecks if plugin_names is empty.
 ---
@@ -419,10 +437,13 @@ function M._check(mods, plugin_names)
     vim.fn.setline(1, 'ERROR: No healthchecks found.')
     return
   end
-  vim.cmd.redraw()
-  vim.print('Running healthchecks...')
 
+  local total_checks = #vim.tbl_keys(healthchecks)
+  local progress_msg = progress_report(total_checks)
+  local check_idx = 1
   for name, value in vim.spairs(healthchecks) do
+    progress_msg('running', check_idx, 'checking %s', name)
+    check_idx = check_idx + 1
     local func = value[1]
     local type = value[2]
     s_output = {}
@@ -475,9 +496,7 @@ function M._check(mods, plugin_names)
     vim.cmd.redraw()
   end
 
-  -- Clear the 'Running healthchecks...' message.
-  vim.cmd.redraw()
-  vim.print('')
+  progress_msg('success', 0, 'checks done')
 
   -- Quit with 'q' inside healthcheck buffers.
   vim._with({ buf = bufnr }, function()