fix(progress): require "source" for progress-message #38514

Problem:
- Progress-events are filtered by "source". But "source" is not required by nvim_echo.
- Without "++nested" (force=false), nvim_echo in an event-handler does not trigger Progress events.
- vim.health does not declare a "source".

Solution:
- Make source mandatory for progress-messages
- Enable ++nested (force=true) by default when firing Progress event.
- Set "source" in vim.health module.

runtime/lua/vim/health.lua

@@ -375,7 +375,7 @@ end
 ---@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' }
+  local progress = { kind = 'progress', source = 'vim.health', title = 'checkhealth' }
 
   return function(status, idx, fmt, ...)
     progress.status = status