fix(scraper): coerce vim.NIL precision to nil before cache write

Problem: vim.json.decode maps JSON null to vim.NIL (userdata), but
cache.set_test_cases validates precision as number|nil, causing a
type error on every scrape where precision is absent.

Solution: guard the precision field when building the callback
table, converting vim.NIL to nil.

lua/cp/scraper.lua

@@ -237,7 +237,7 @@ function M.scrape_all_tests(platform, contest_id, callback)
             memory_mb = ev.memory_mb or 0,
             interactive = ev.interactive or false,
             multi_test = ev.multi_test or false,
-            precision = ev.precision,
+            precision = ev.precision ~= vim.NIL and ev.precision or nil,
             problem_id = ev.problem_id,
           })
         end