fix(parse): cast os.date('*t') return to osdate (#3)

Problem: LuaLS types os.date('*t') as string|osdate, causing type
errors when accessing .year, .month, .day, .wday fields in
is_valid_date and resolve_date.

Solution: add --[[@as osdate]] casts on both os.date('*t') calls.

lua/pending/parse.lua

@@ -21,7 +21,7 @@ local function is_valid_date(s)
     return false
   end
   local t = os.time({ year = y, month = m, day = d })
-  local check = os.date('*t', t)
+  local check = os.date('*t', t) --[[@as osdate]]
   return check.year == y and check.month == m and check.day == d
 end
 
@@ -44,7 +44,7 @@ local weekday_map = {
 ---@return string|nil
 function M.resolve_date(text)
   local lower = text:lower()
-  local today = os.date('*t')
+  local today = os.date('*t') --[[@as osdate]]
 
   if lower == 'today' then
     return os.date('%Y-%m-%d', os.time({ year = today.year, month = today.month, day = today.day })) --[[@as string]]