feat(race): add supports_countdown to ContestListResult

Problem: `:CP race` on platforms without future contests (CSES, USACO)
wastes time fetching the full contest list only to discover there is
no `start_time`. The error message is also uninformative.

Solution: Add `supports_countdown` bool to `ContestListResult` (default
`True`). CSES and USACO set it to `False`. Cache the flag per-platform
so subsequent calls skip the fetch entirely. `race.lua` checks the
cached value first, then the scraper result, and shows
`"<Platform> does not support :CP race"` instead of a generic error.

lua/cp/cache.lua

@@ -392,7 +392,8 @@ end
 
 ---@param platform string
 ---@param contests ContestSummary[]
-function M.set_contest_summaries(platform, contests)
+---@param opts? { supports_countdown?: boolean }
+function M.set_contest_summaries(platform, contests, opts)
   cache_data[platform] = cache_data[platform] or {}
   for _, contest in ipairs(contests) do
     cache_data[platform][contest.id] = cache_data[platform][contest.id] or {}
@@ -405,9 +406,22 @@ function M.set_contest_summaries(platform, contests)
     end
   end
 
+  if opts and opts.supports_countdown ~= nil then
+    cache_data[platform].supports_countdown = opts.supports_countdown
+  end
+
   M.save()
 end
 
+---@param platform string
+---@return boolean?
+function M.get_supports_countdown(platform)
+  if not cache_data[platform] then
+    return nil
+  end
+  return cache_data[platform].supports_countdown
+end
+
 ---@param platform string
 ---@param contest_id string
 ---@return integer?

lua/cp/pickers/init.lua

@@ -48,8 +48,10 @@ function M.get_platform_contests(platform, refresh)
       { level = vim.log.levels.INFO, override = true, sync = true }
     )
 
-    local contests = scraper.scrape_contest_list(platform)
-    cache.set_contest_summaries(platform, contests)
+    local result = scraper.scrape_contest_list(platform)
+    local contests = result and result.contests or {}
+    local sc = result and result.supports_countdown
+    cache.set_contest_summaries(platform, contests, { supports_countdown = sc })
     picker_contests = cache.get_contest_summaries(platform)
 
     logger.log(

lua/cp/race.lua

@@ -35,20 +35,36 @@ function M.start(platform, contest_id, language)
   end
 
   cache.load()
+
+  local display = constants.PLATFORM_DISPLAY_NAMES[platform] or platform
+  local cached_countdown = cache.get_supports_countdown(platform)
+  if cached_countdown == false then
+    logger.log(('%s does not support :CP race'):format(display), { level = vim.log.levels.ERROR })
+    return
+  end
+
   local start_time = cache.get_contest_start_time(platform, contest_id)
 
   if not start_time then
-    logger.log('Fetching contest list...', { level = vim.log.levels.INFO, override = true })
-    local contests = scraper.scrape_contest_list(platform)
-    if contests and #contests > 0 then
-      cache.set_contest_summaries(platform, contests)
-      start_time = cache.get_contest_start_time(platform, contest_id)
+    logger.log('Fetching contest list...', { level = vim.log.levels.INFO, override = true, sync = true })
+    local result = scraper.scrape_contest_list(platform)
+    if result then
+      local sc = result.supports_countdown
+      if sc == false then
+        cache.set_contest_summaries(platform, result.contests or {}, { supports_countdown = false })
+        logger.log(('%s does not support :CP race'):format(display), { level = vim.log.levels.ERROR })
+        return
+      end
+      if result.contests and #result.contests > 0 then
+        cache.set_contest_summaries(platform, result.contests, { supports_countdown = sc })
+        start_time = cache.get_contest_start_time(platform, contest_id)
+      end
     end
   end
 
   if not start_time then
     logger.log(
-      ('No start time found for %s contest %s'):format(
+      ('No start time found for %s contest "%s"'):format(
         constants.PLATFORM_DISPLAY_NAMES[platform] or platform,
         contest_id
       ),

lua/cp/scraper.lua

@@ -257,9 +257,12 @@ function M.scrape_contest_list(platform)
       ),
       { level = vim.log.levels.ERROR }
     )
-    return {}
+    return nil
   end
-  return result.data.contests
+  return {
+    contests = result.data.contests,
+    supports_countdown = result.data.supports_countdown ~= false,
+  }
 end
 
 ---@param platform string

scrapers/cses.py

@@ -228,9 +228,13 @@ class CSESScraper(BaseScraper):
         cats = parse_categories(html)
         if not cats:
             return ContestListResult(
-                success=False, error=f"{self.platform_name}: No contests found"
+                success=False,
+                error=f"{self.platform_name}: No contests found",
+                supports_countdown=False,
             )
-        return ContestListResult(success=True, error="", contests=cats)
+        return ContestListResult(
+            success=True, error="", contests=cats, supports_countdown=False
+        )
 
     async def login(self, credentials: dict[str, str]) -> LoginResult:
         username = credentials.get("username", "")

scrapers/models.py

@@ -50,6 +50,7 @@ class MetadataResult(ScrapingResult):
 
 class ContestListResult(ScrapingResult):
     contests: list[ContestSummary] = Field(default_factory=list)
+    supports_countdown: bool = True
 
     model_config = ConfigDict(extra="forbid")
 

scrapers/usaco.py

@@ -337,10 +337,16 @@ class USACOScraper(BaseScraper):
                     contests.extend(await coro)
 
             if not contests:
-                return self._contests_error("No contests found")
-            return ContestListResult(success=True, error="", contests=contests)
+                return ContestListResult(
+                    success=False, error="No contests found", supports_countdown=False
+                )
+            return ContestListResult(
+                success=True, error="", contests=contests, supports_countdown=False
+            )
         except Exception as e:
-            return self._contests_error(str(e))
+            return ContestListResult(
+                success=False, error=str(e), supports_countdown=False
+            )
 
     async def stream_tests_for_category_async(self, category_id: str) -> None:
         month_year, division = _parse_contest_id(category_id)